Programmable Logic Controller

Learning System for Automation and Technology

Programmable logic controllers Basic level TP301 – Textbook

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Authorised applications and liability The Learning System for Automation and Technology has been developed and prepared exclusively for training in the field of automation. The training organization and/or trainee shall ensure that the safety precautions described in the accompanying Technical documentation are fully observed. Festo Didactic hereby excludes any liability for injury to trainees, to the training organization and/or to third parties occurring as a result of the use or application of the station outside of a pure training situation, unless caused by premeditation or gross negligence on the part of Festo Didactic. Order No.: Description: Designation: Edition: Layout: Graphics: Authors:

093311 SPS LB GS D.LB-TP301–1-GB 08/2002 28.08.2002, OCKER Ingenieurbüro D. Schwarzenberger, OCKER Ingenieurbüro R. Bliesener, F.Ebel, C.Löffler, B. Plagemann, H.Regber, E.v.Terzi, A. Winter

© Copyright by Festo Didactic GmbH & Co., D-73770 Denkendorf 2002 The copying, distribution and utilization of this document as well as the communication of its contents to others without expressed authorization is prohibited. Offenders will be held liable for the payment of damages. All rights reserved, in particular the right to carry out patent, utility model or ornamental design registrations. Parts of this training documentation may be duplicated, solely for training purposes, by persons authorised in this sense.

TP301 • Festo Didactic

B-III

Preface The programmable logic controller represents a key factor in industrial automation. Its use permits flexible adaptation to varying processes as well as rapid fault finding and error elimination. This textbook explains the design of a programmable logic controller and its interaction with peripherals. One of the main focal points of the textbook deals with the new international standard for PLC programming, the EN 61131-3 (IEC-61131-3). This standard takes into account expansions and developments, for which no standardised language elements existed hitherto. The aim of this new standard is to standardise the design, functionality and the programming of a PLC in such a way as to enable the user to easily operate with different systems. In the interest of continual further improvement, all readers of this book are invited to make contributions by way suggestions, ideas and constructive criticism. August 2002

TP301 • Festo Didactic

The authors

B-IV

TP301 • Festo Didactic

B-V

Table of Contents Chapter 1 Automating with a PLC

B-1

1.1

Introduction

B-1

1.2

Areas of application of a PLC

B-2

1.3

Basic design of a PLC

B-5

1.4

The new PLC standard EN 61131 (IEC 61131)

B-8

Chapter 2 Fundamentals

B-11

2.1

The decimal number system

B-11

2.2

The binary number system

B-11

2.3

The BCD code

B-13

2.4

The hexadecimal number system

B-13

2.5

Signed binary numbers

B-14

2.6

Real numbers

B-14

2.7

Generation of binary and digital signals

B-15

Chapter 3 Boolean operations

B-19

3.1

Basic logic functions

B-19

3.2

Further logic operations

B-23

3.3

Establishing switching functions

B-25

3.4

Simplification of logic functions

B-28

3.5

Karnaugh-Veitch diagram

B-30

TP301 • Festo Didactic

B-VI

Chapter 4 Design and mode of operation of a PLC

B-33

4.1

Structure of a PLC

B-33

4.2

Central control unit of a PLC

B-35

4.3

Function mode of a PLC

B-37

4.4

Application program memory

B-39

4.5

Input module

B-41

4.6

Output module

B-43

4.7

Programming device/Personal computer

B-45

Chapter 5 Programming of a PLC

B-47

5.1

Systematic solution finding

B-47

5.2

EN 61131-3 (IEC 61131-3) structuring resources

B-50

5.3

Programming languages

B-54

Chapter 6 Common elements of programming languages

B-57

6.1

Resources of a PLC

B-57

6.2

Variables and data types

B-60

6.3

Program

B-70

Chapter 7 Function block diagram

B-85

7.1

Elements of function block diagram

B-85

7.2

Evaluation of networks

B-85

7.3

Loop structures

B-87

Chapter 8 Ladder diagram

B-89

8.1

Elements of ladder diagram

B-89

8.2

Functions and function blocks

B-92

8.3

Evaluation of current rungs

B-93

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Chapter 9 Instruction list

B-95

9.1

Instructions

B-95

9.2

Operators

B-96

9.3

Functions and function blocks

B-97

Chapter 10 Structured text

B-99

10.1

Expressions

B-99

10.2

Statements

B-101

10.3

Selection statements

B-103

10.4

Iteration statements

B-106

Chapter 11 Sequential function chart

B-111

11.1

Introduction

B-111

11.2

Elements of sequential function chart

B-111

11.3

Transitions

B-120

11.4

Steps

B-123

11.5

Example

B-135

Chapter 12 Logic control systems

B-139

12.1

What is a logic control system

B-139

12.2

Logic control systems without latching properties

B-139

12.3

Logic control systems with memory function

B-145

12.4

Edge evaluation

B-148

Chapter 13 Timers

B-153

13.1

Introduction

B-153

13.2

Pulse timer

B-154

13.3

Switch-on signal delay

B-156

13.4

Switch-off signal delay

B-158

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Chapter 14 Counter

B-161

14.1

Counter functions

B-161

14.2

Incremental counter

B-161

14.3

Decremental counter

B-165

14.4

Incremental/decremental counter

B-167

Chapter 15 Sequence control systems

B-169

15.1

What is a sequence control system

B-169

15.2

Function chart to IEC 60848

B-169

Chapter 16 Commissioning and operational safety of a PLC

B-175

16.1

Commissioning

B-175

16.2

Operational safety of a PLC

B-177

Chapter 17 Communication

B-183

17.1

The need for communication

B-183

17.2

Data transmission

B-183

17.3

Interfaces

B-184

17.4

Communication in the field area

B-185

A

Bibliography of illustrations

B-187

B

Bibliography of literature

B-189

C

Guidelines and standards

B-191

D

Glossary

B-193

E

Index

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Appendix

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B-1 Chapter 1

The PLC in automation technology 1.1 Introduction The first Programmable Logic Controller (PLC) was developed by a group of engineers at General Motors in 1968, when the company were looking for an alternative to replace complex relay control systems. The new control system had to meet the following requirements:


Simple programming




Program changes without system intervention (no internal rewiring)




Smaller, cheaper and more reliable than corresponding relay control systems




Simple, low cost maintenance

Subsequent development resulted in a system, which enabled the simple connection of binary signals. The requirements as to how these signals were to be connected were specified in the control program. With the new systems it became possible for the first time to plot signals on a screen and to file these in electronic memories. Since then, three decades have passed, during which the enormous progress made in the development of microelectronics did not stop short of programmable logic controllers. For instance, even if program optimisation and thus a reduction of required memory capacity initially still represented an important key task for the programmer, nowadays this is hardly of any significance. Moreover, the range of functions has grown considerably. 15 years ago, process visualisation, analogue processing or even the use of a PLC as a controller, were considered as Utopian. Nowadays, the support of these functions forms an integral part of many PLCs. The following pages in this introductory chapter outline the basic design of a PLC together with the currently most important tasks and applications.

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B-2 Chapter 1

1.2 Areas of application of a PLC Every system or machine has a controller. Depending on the type of technology used, controllers can be divided into pneumatic, hydraulic, electrical and electronic controllers. Frequently, a combination of different technologies is used. Furthermore, differentiation is made between hard-wired programmable (e.g. wiring of electro-mechanical or electronic components) and programmable logic controllers. The first is used primarily in cases, where any reprogramming by the user is out of the question and the job size warrants the development of a special controller. Typical applications for such controllers can be found in automatic washing machines, video cameras, and cars. However, if the job size does not warrant the development of a special controller or if the user is to have the facility of making simple or independent program changes, or of setting timers and counters, then the use of a universal controller, where the program is written to an electronic memory, is the preferred option. The PLC represents such a universal controller. It can be used for different applications and, via the program installed in its memory, provides the user with a simple means of changing, extending and optimising control processes.

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B-3 Chapter 1

Fig. B1.1: Example of a PLC application

The original task of a PLC involved the interconnection of input signals according to a specified program and, if "true", to switch the corresponding output. Boolean algebra forms the mathematical basis for this operation, which recognises precisely two defined statuses of one variable: "0" and "1" (see also chapter 3). Accordingly, an output can only assume these two statuses. For instance, a connected motor could therefore be either switched on or off, i.e. controlled. This function has coined the name PLC: Programmable logic controller, i.e. the input/output behaviour is similar to that of an electromagnetic relay or pneumatic switching valve controller; the program is stored in an electronic memory. However, the tasks of a PLC have rapidly multiplied: Timer and counter functions, memory setting and resetting, mathematical computing operations all represent functions, which can be executed by practically any of today’s PLCs.

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B-4 Chapter 1

The demands to be met by PLC’s continued to grow in line with their rapidly spreading usage and the development in automation technology. Visualisation, i.e. the representation of machine statuses such as the control program being executed, via display or monitor. Also controlling, i.e. the facility to intervene in control processes or, alternatively, to make such intervention by unauthorised persons impossible. Very soon, it also became necessary to interconnect and harmonise individual systems controlled via PLC by means of automation technology. Hence a master computer facilitates the means to issue higher-level commands for program processing to several PLC systems The networking of several PLCs as well as that of a PLC and master computer is effected via special communication interfaces. To this effect, many of the more recent PLCs are compatible with open, standardised bus systems, such as Profibus to EN 50170. Thanks to the enormously increased performance capacity of advanced PLCs, these can even directly assume the function of a master computer. At the end of the seventies, binary inputs and outputs were finally expanded with the addition of analogue inputs and outputs, since many of today’s technical applications require analogue processing (force measurement, speed setting, servo-pneumatic positioning systems). At the same time, the acquisition or output of analogue signals permits an actual/setpoint value comparison and as a result the realisation of automatic control engineering functions, a task, which widely exceeds the scope suggested by the name (programmable logic controller). The PLCs currently on offer in the market place have been adapted to customer requirements to such an extent that it has become possible to purchase an eminently suitable PLC for virtually any application. As such, miniature PLCs are now available with a minimum number of inputs/outputs starting from just a few hundred Pounds. Also available are larger PLCs with 28 or 256 inputs/outputs. Many PLCs can be expanded by means of additional input/output, analogue, positioning and communication modules. Special PLCs are available for safety technology, shipping or mining tasks. Yet further PLCs are able to process several programs simultaneously – (multitasking). Finally, PLCs are coupled with other automation components, thus creating considerably wider areas of application.

TP301 • Festo Didactic

B-5 Chapter 1

Fig. B1.2: Example of a PLC: Festo IPC PS1 Professional

1.3 Basic design of a PLC The term ’programmable logic controller’ is defined as follows by EN 61131-1 (IEC 61131-1): “ A digitally operating electronic system, designed for use in an industrial environment, which uses a programmable memory for the internal storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic, to control, through digital or analogue inputs and outputs, various types of machines or processes. Both the PC and its associated peripherals are designed so that they can be easily integrated into an industrial control system and easily used in all their intended functions." A programmable logic controller is therefore nothing more than a computer, tailored specifically for certain control tasks.

TP301 • Festo Didactic

B-6 Chapter 1

Fig. B1.3 illustrates the system components of a PLC. Fig. B1.3: System components of a PLC

PLC-program

Input module

Sensors

Central control unit

Output module

Actuators

The function of an input module is to convert incoming signals into signals, which can be processed by the PLC, and to pass these to the central control unit. The reverse task is performed by an output module. This converts the PLC signal into signals suitable for the actuators. The actual processing of the signals is effected in the central control unit in accordance with the program stored in the memory. The program of a PLC can be created in various ways: via assemblertype commands in ’statement list’, in higher-level, problem-oriented languages such as structured text or in the form of a flow chart such as represented by a sequential function chart. In Europe, the use of function block diagrams based on function charts with graphic symbols for logic gates is widely used. In America, the ’ladder diagram’ is the preferred language by users. Depending on how the central control unit is connected to the input and output modules, differentiation can be made between compact PLCs (input module, central control unit and output module in one housing) or modular PLCs.

TP301 • Festo Didactic

B-7 Chapter 1

Fig. B1.4 shows the FX0 controller by Mitsubishi and the IPC FEC Standard controller by Festo as an Example Fig. B1.4: Compact-PLC (Mitsubishi FX0, Festo IPC FEC Standard), modular PLC (Siemens S7-300)

Modular PLCs may be configured individually. The modules required for the practical application – apart from digital input/output modules, which can, for instance, include analogue, positioning and communication modules – are inserted in a rack, where individual modules are linked via a bus system. This type of design is also known as series technology. Two examples of modular PLCs are shown in figs. B1.2 and B1.4. These represent the modular system IPC PS1 Professional by Festo and the new S7-300 series by Siemens.

TP301 • Festo Didactic

B-8 Chapter 1

A wide range of variants exists, particularly in the case of more recent PLCs. These include both modular as well as compact characteristics and important features such as spacing saving, flexibility and scope for expansion. The card format PLC is a special type of modular PLC, developed during the last few years. With this type, individual or a number of printed circuit board modules are in a standardised housing. The hardware design for a programmable logic controller is such that it is able to withstand typical industrial environments as regard signal levels, heat, humidity, and fluctuations in current supply and mechanical impact. 1.4 The new PLC standard EN 61131 (IEC 61131) Previously valid PLC standards focussing mainly on PLC programming were generally geared to current state of the art technology in Europe at the end of the seventies. This took into account non-networked PLC systems, which primarily execute logic operations on binary signals. Previously, no equivalent, standardised language elements existed for the PLC developments and system expansions made in the eighties, such as processing of analogue signals, interconnection of intelligent modules, networked PLC systems etc. Consequently, PLC systems by different manufacturers required entirely different programming. Since 1992, an international standard now exists for programmable logic controllers and associated peripheral devices (programming and diagnostic tools, testing equipment, man-to-machine interfaces etc.). In this context, a device configured by the user and consisting of the above components is known as a PLC system.

TP301 • Festo Didactic

B-9 Chapter 1

The new EN 61131 (IEC 61131) standard consists of five parts:


Part 1: General information




Part 2: Equipment requirements and tests




Part 3: Programming languages




Part 4: User guidelines (in preparation with IEC)




Part 5: Messaging service specification (in preparation with IEC)

Parts 1 to 3 of this standard were adopted unamended as European Standard EN 61 131, Parts 1 to 3. The purpose of the new standard was to define and standardise the design and functionality of a PLC and the languages required for programming to the extent where users were able to operate using different PLC systems without any particular difficulties. The next chapters will be dealing with this standard in greater detail. However, for the moment the following information should suffice:


The new standard takes into account as many aspects as possible regarding the design, application and use of PLC systems.




The extensive specifications serve to define open, standardised PLC systems.




Manufacturers must conform to the specifications of this standard both with regard to purely technical requirements for the PLC as well as the programming of controllers.




Any variations must be fully documented for the user.

After initial reservations, a large group of interested people (PLCopen) has been formed to support this standard. A large number of major PLC suppliers are members of the association, i.e. ABB, GE Fanuc, Mitsubishi Electric, Moeller, OMRON, Schneider Electric, Siemens. A large number of the members of the association offer control and programming systems conforming to EN 61131 (IEC 61131). In the future, languages in accordance with IEC 61131 will not only dominate PLC programming, but rather industrial automation in its entirety.

TP301 • Festo Didactic

B-10 Chapter 1

TP301 • Festo Didactic

B-11 Chapter 2

Fundamentals 2.1 The decimal number system Characteristic of the decimal number system used in general is the linear array of digits and their significant placing. The number 4344, for instance, can be represented as follows: 4344 = 4 x 1000 + 3 x 100 + 4 x 10 + 4 x 1 Number 4 on the far left is of differing significance to that of number 4 on the far right. The basis of the decimal number system is the availability of 10 different digits (decimal: originating from the Latin ’decem’ = 10 ). These 10 different digits permit counting from 0 to 9. If counting is to exceed the number 9, this constitutes a carry over to the next place digit. The significance of this place is 10, and the next carry over takes place when 99 is reached. The number 71.718.711 is to be used as an example: 10

7

10 7

6

10 1

5

10 7

4

10 1

3

10 8

2

10 7

1

10 1

0

Example 1

As can be seen from the above, the significance of the "7" on the far left is 70.000.000 = 70 million, whereas the significance of the "7" in the third place from the right is 700. The digit on the far right is referred to as the least significant digit, and the digit on the far left as the most significant digit. Any number system can be configured on the basis of this example, the fundamental structure can be applied to number systems of any number of digits. Consequently, any computing operations and computing methods which use the decimal number system can be applied with other number systems. 2.2 The binary number system We are indebted to Leibnitz, who applied the structures of the decimal number system to two-digit calculation. As long ago as 1679, this created the premises essential for the development of the computer, since electrical voltage or electrical current only permits a calculation using just two values: e.g. "current on", "current off". These two values are represented in the form of digits: "1" and "0".

TP301 • Festo Didactic

B-12 Chapter 2

If one were to be limited to exactly 2 digits per place of a number, then a number system would be configured as follows: Example

27 = 128

26 = 64

25 = 32

24 = 16

1

0

1

1

23 = 8

22 = 4

0

21 = 2

0

20 = 1

0

1

The principle is exactly the same as that of the method used to create a decimal number. However, only two digits are available, which is why the significant place is not calculated to the base 10x, but to the base 2x. Hence the lowest significant number on the far right is0 = 1, and of the next place 21 = 2 etc. Because of the exclusive use of two digits, this number system is known as the binary or also the dual number system. Up to a maximum of 28 – 1 = 256 – 1 = 255 can be calculated number 1111 11112.

with

eight

places,

which

would

be

the

The individual places of the binary number system can adopt one of the two digits 0 or 1. This smallest possible unit of the binary system is termed 1 bit. In the above example, a number consisting of 8 bits, i.e. one byte, has been configured (in a computer using 8 electrical signals representing either "voltage available" or "voltage not available" or "current on" or "current off".) The number considered, 1011 00012, assumes the decimal value 17710. Example

1 x 27 = 128

0 x 26

1 x 25

1 x 24

+ 32

+ 16

0 x 23

0 x 22

0 x 21

1 x 20 1

= 177

TP301 • Festo Didactic

B-13 Chapter 2

2.3 The BCD code For people used to dealing with the decimal system, binary numbers are difficult to read. For this reason, a more easily readable numeral representation was introduced; i.e. the binary coded decimal notation, the socalled BCD code (binary coded decimal). With this BCD code, each individual digit of the decimal number system is represented by a corresponding binary number: 010

0000BCD

110

0001BCD

210

0010BCD

310

0011BCD

410

0100BCD

510

0101BCD

610

0110BCD

710

0111BCD

810

1000BCD

910

1001BCD

4 digits in binary notation are therefore required for the 10 digits in the decimal system. The discarded place (in binary notation, the numbers 0 to 15 may be represented with 4 digits) is accepted for the sake of clarity. The decimal number 7133 is thus represented as follows in the BCD code: 0111 0001 0011 0011BCD 16 bits are therefore required to represent a four digit decimal number in the BCD code. BCD coded numbers are often used for seven segment displays and coding switches. 2.4 The hexadecimal number system The use of binary numbers is often difficult for the uninitiated and the use of the BCD code takes up a lot of space. This is why the octal and the hexadecimal system were developed. Three digits are always combined in the case of the octal number system. This permits counting from 0 to 7, i.e. counting in "eights".

TP301 • Festo Didactic

Table B2.1: Representation of decimal numbers in BCD code

B-14 Chapter 2

Alternatively, 4 bits are combined with the hexadecimal number system. 4 bits permit the representation of the numbers 0 to 15, i.e. counting in "sixteens". The digits 0 to 9 are used to represent these numbers in digits, followed by the letters A, B, C, D, E and F where A = 10, B = 11, C = 12, D = 13, E = 14 and F = 15. The significant place of the individual digits is to the base 16. Example

163 = 4096

162 = 256

161 = 16

160 = 1

8

7

B

C

The number 87BC16 given as an example therefore reads as follows: 8 x 163 + 7 x 162 + 11 x 161 + 12 x 160 = 34 74810 2.5 Signed binary numbers Up to now, we have dealt solely with whole positive numbers, not taking into account negative numbers. To enable working with these negative numbers, it was decided that the most significant bit on the far left of a binary number is to be used to represent the preceding sign: "0" thus corresponds to "+" and "1" corresponds to "–". Hence 1111 11112 = -12710 and 0111 11112 = +12710 Since the most significant bit has been used, one bit less is available for the representation of a signed number. In the field of data processing, the use of so-called compliment representation for the expression of negative numbers has proven useful. The following range of values is obtained for the representation of a 16 digit binary number: Example

Integer

Range of values

unsigned

0 to 65535

signed

-32768 to +32767

2.6 Real numbers Although it is now possible for whole positive and whole signed numbers to be represented with 0 or 1, there is still the need for points or real numbers. In order to represent a real number in computer binary notation, the number is split into two groups, a power of ten and a multiplication factor. This is also known as the scientific representation of digits.

TP301 • Festo Didactic

B-15 Chapter 2

The number 27,3341 is thus converted into 273 341 x 10-4. Two wholesigned numbers are therefore required for a real number to be represented in a computer. 2.7 Generation of binary and digital signals As has already become clearly apparent in the previous section, all computers and as such all PLCs operate using binary or digital signals. By binary signal, we understand a signal, which recognises only two defined values. Fig. B2.1: Binary signal

1

0

t

These values are termed "0" or "1", the terms "low" and "high" are also used. The signals can be very easily realised with contacting components. An actuated normally open contact corresponds to a logic 1signal and an unactuated one to a logic 0-signal. When working with contactless components, this can give rise to certain tolerance bands. For this reason, certain voltage ranges have been defined as logic 0 or logic 1 ranges. Fig. B2.2: Voltage ranges

V 30 1 - range

11 5 0 -3

TP301 • Festo Didactic

0 - range t

B-16 Chapter 2

EN 61131-2 (IEC 61131-2) defines a value range of -3 V to 5 V as logic 0-signal, and 11 V to 30 V as logic 1-signal (for contactless sensors). This is binding for PLCs, whose device technology is to conform to EN 61131-2 (IEC 61131-2). In current practice, however, other voltage ranges can often be found for logic 0- and 1-signal. Widely used are: -30 V to +5 V as logic 0, 13 V to 30 V as logic 1. Unlike binary signals, digital signals can assume any value. These are also referred to as value stages. A digital signal is thus defined by any number of value stages. The change between these is non-sequential. The following illustration shows three possible methods of converting an analogue signal into a digital signal. Fig. B2.3: Conversion of an analogue signal into a digital signal

V 6 5

Analogues Signal

Digital Signal on 0,5V basis

Digital Signal on 1V basis

4 3 2

Digital Signal on 3V basis

1 0

t

Digital signals may be formed from analogue signals. This method is for instance used for analogue processing via PLC. Accordingly, the analogue input signal within a range of 0 to 10 V is reduced into a series of step values. Depending on the quality of the PLC and the possible step height set, the digital signal would thus be able to operate in steps of value of 0.1 V, 0.01 V or 0.001 V. Naturally, the smallest range is selected in this instance in order for the analogue signal to be reproduced as accurately as possible.

TP301 • Festo Didactic

B-17 Chapter 2

One simple example of an analogue signal is pressure, which is measured and displayed by a pressure gauge. The pressure signal may assume any intermediate value between its minimum and maximum values. Unlike the digital signal, it changes continually. In the case of the processing of analogue values via a PLC, as described, analogue voltage signals are evaluated and converted. On the other hand, digital signals can be formed by adding together a certain number of binary signals. In this way, again as described in the above paragraph, it is also possible to generate a digital signal with 256 step values. Bit No.

7

6

5

4

3

2

1

0

Digital value

Example 1

1

0

1

1

1

0

1

1

187

Example 2

0

0

1

1

0

0

1

1

51

Example 3

0

0

0

0

0

0

0

0

0

This process is for instance used to implement timer and counter functions.

TP301 • Festo Didactic

Example

B-18 Chapter 2

TP301 • Festo Didactic

B-19 Chapter 3

Boolean operations 3.1. Basic logic functions As described in the previous chapter, any computer and equally any PLC operates using the number system to the base 2. This also applies to the octal (23) and the hexadecimal systems (24). The individual variable can therefore assume only two values, "0" or "1". Special algorithms have been introduced to be able to link these variables – the socalled boolean algebra. This can be clearly represented by means of electrical contacts. Negation (NOT function) The push button shown represents a normally closed contact. When this is unactuated, lamp H1 is illuminated, whereas in the actuated state, lamp H1 goes off. Fig. B3.1: Circuit diagram

24V S1 (I)

H1 (O) 0V

Push button S1 acts as signal input, the lamp forms the output. The actual status can be recorded in a truth table: I

O

0

1

1

0

The boolean equation is therefore as follows:

I = O (read: Not I equals O)

TP301 • Festo Didactic

Fig. B3.2: Truth table

B-20 Chapter 3

The logic symbol is: Fig. B3.3: NOT function 1

I

O

If 2 negations are switched in succession, then these cancel one another. Fig. B3.4: 2 logic NOT functions

I=I 1

I

I

1

I

Conjunction (AND-function) If two normally open contacts are switched in series, the actuated lamp is illuminated only if both push buttons are actuated. Fig. B3.5: Circuit diagram

24V S1 (I1) S2 (I2) H1 (O) 0V

TP301 • Festo Didactic

B-21 Chapter 3

I1

I2

O

0

0

0

0

1

0

1

0

0

1

1

1

Fig. B3.6 Truth table

The truth table assigns the conjunction. The output assumes 1 only if both input 1 and input 2 produce a "1"-signal. This is referred to as an AND operation, which is represented as follows as an equation:

I1 ∧ I2 = O Fig. B3.7 AND function I1 I2

& O

In addition, the following algorithms apply for the conjunction:

a∧0 =0 a ∧1= a a∧a =0 a∧a =a

TP301 • Festo Didactic

B-22 Chapter 3

Disjunction (OR-Function) Another basic logic function is OR. If the 2 normally open contacts are switched in parallel, then the lamp is illuminated whenever a least one push button is pressed. Fig. B3.8: Circuit diagram

24V S1 (I1)

S2 (I2)

H1 (O) 0V

Fig. B3.9 Truth table

Fig. B3.10: OR function

I1

I2

O

0

0

0

0

1

1

1

0

1

1

1

1

I1 I2

>=1 O

The logic operation is written in the form of the following equation:

I1 ∨ I2 = O The following algorithms also apply for the OR-operation:

b∨0 =b b ∨1= 1 b∨b =b b∨ b =1

TP301 • Festo Didactic

B-23 Chapter 3

3.2 Further logic operations The electrical realisation of a NOT-/AND-/OR-operation has already been described in section B3.1. Each of these operations can of course also be realised pneumatically or electronically. Boolean algebra also recognises the following logic operations. The following table provides an overview of these. Table B3.1: Logic connections Name

Equation

Truth table

log. symbols

pneumatic realisation

I

O

elektr. realisation

elektron. realisation

I I

0 1

R

O

1

Negation

I

O

I

O

R

O

O

I O

I1 I2 O I1

>

Conjuction

I2 = O

0 1 1

R

I1 I2

O O

O I1

I1

>

Disjunction

I2 = O

TP301 • Festo Didactic

0 0 1

I2

I2

>=1 R

B-24 Chapter 3

Table B3.1: Logic connections (continuation) Name

Equation

Truth table

log. symbols

pneumatic realisation

elektr. realisation

I1

I1 I2 O Antivalence (exclusive OR)

I1

I2 I2

I1 I2 O Equivalence

elektron. realisation

I1

I1

O

I1

= I2

O

O

O

I1

NAND

K1 O

I1 I2 K1

I2

O

NOR

O

K1 O

>=1 K1

O

TP301 • Festo Didactic

B-25 Chapter 3

3.3 Establishing switching functions Deriving boolean equations from the truth table Often, the logic operations shown in the previous section are not enough to adequately describe a status in control technology. Very often, there is a combination of different logic operations. The logic connection in the form of a boolean equation can be easily established from the truth table. The example below should clarify this: Sorting station task Various parts for built-in kitchens are to be machined in a production system (milling and drilling machine). The wall and door sections for certain types of kitchen are to be provided with different drill holes. Sensors B1 to B4 are intended for the detection of the holes. Fig. B3.11: Sorting station

1A1 B1

B4 B3

B2

Parts with the following hole patterns are for the ’Standard’ kitchen type. These parts are to be advanced via the double-acting cylinder 1.0.

TP301 • Festo Didactic

B-26 Chapter 3

Fig. B3.12: Hole pattern parts

a

a

d

b

c

a

d

b

d

d

d

a

c

b

d

Assuming that a drilled hole is read as a 1-signal, the following truth table results. Fig. B3.13 Truth table

a

b

c

d

y

0

0

0

0

0

0

0

0

1

1

0

0

1

0

0

0

0

1

1

0

0

1

0

0

0

0

1

0

1

1

0

1

1

0

0

0

1

1

1

0

1

0

0

0

0

1

0

0

1

1

1

0

1

0

0

1

0

1

1

1

1

1

0

0

0

1

1

0

1

1

1

1

1

0

0

1

1

1

1

1

TP301 • Festo Didactic

B-27 Chapter 3

Two options are available in order to derive the logic equation from this table, which lead to two different expressions. The same result is obtained, of course, since the same circumstances are described. Standard form, disjunctive In the disjunctive standard form, all conjunctions (AND-operations) of input variables with the result 1, are carried out as a disjunctive operation (OR-operation). With signal status 0, the input variable is carried out as a negated operation and with signal status 1 as a non-negated operation. In the case of the example given, the logic operation is therefore as follows:

(

) (

) (

)

y = a∧b∧c ∧d ∨ a∧b∧c ∧d ∨ a∧b∧c ∧d ∨

(a ∧ b ∧ c ∧ d) ∨ (a ∧ b ∧ c ∧ d) ∨ (a ∧ b ∧ c ∧ d) Conjunctive standard form In the conjunctive standard form, all disjunctions (OR-operations) of the input variable producing the result 0, are carried out as a conjunctive operation (AND-operation). In contrast with the disjunctive standard form, in this instance, the input variable is negated with signal status "1" and a non-negated operation carried out with signal status "0". y=

(a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧

(a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d) ∧ (a ∨ b ∨ c ∨ d)

TP301 • Festo Didactic

B-28 Chapter 3

3.4 Simplification of logic functions Both equations for the example given are rather extensive, with that of the conjunctive standard form being even longer still. This defines the criteria for using the disjunctive or conjunctive standard from: The decision is made in favour of the shorter form of the equation. In this case, the disjunctive standard form.

(

) (

) (

)

y = a∧b∧c ∧d ∨ a∧b∧c ∧d ∨ a∧b∧c ∧d ∨

(a ∧ b ∧ c ∧ d) ∨ (a ∧ b ∧ c ∧ d) ∨ (a ∧ b ∧ c ∧ d) This expression may be simplified with the help of a boolean algorithm. The most important rules in boolean algebra are shown below:

a∨0 = a

a∧0 =0

a ∨1= 1

a ∧1= a

a∨a =a

a∧a =a

a∨a =1

a∧a =0

Commutative law

a∨b =b∨a

a∧b =b∧a

Associative law

a ∨ b ∨ c = a ∨ (b ∨ c ) = (a ∨ b ) ∨ c a ∧ b ∧ c = a ∧ (b ∧ c ) = (a ∧ b ) ∧ c

TP301 • Festo Didactic

B-29 Chapter 3

Distributive law

a ∧ (b ∨ c ) = (a ∧ b ) ∨ (a ∧ c )

a ∨ (b ∧ c ) = (a ∨ b ) ∧ (a ∨ c )

De Morgan’s rule

a∨b = a∧b

a∧b = a∨b

Reduction rule

a∨ a ∧b = a∨b

Applied to the above example, the following result is obtained: y = abcd ∨ abcd ∨ abcd ∨ abcd ∨ abcd ∨ abcd

(

= abcd ∨ abcd ∨ abcd ∨ abcd ∨ abd c ∨ c

(

)

)

= acd b ∨ b ∨ abd(c ∨ c ) ∨ abd = acd ∨ abd ∨ abd

(

= acd ∨ ad b ∨ b

(

)

)

= ac ∨ a d

(

)

= c∨ad = cd ∨ ad For reasons of clarity, the AND-operation symbol “∧”has been omitted in the individual expressions. The basic principle of simplification is in the factoring out of variables and reducing to defined expressions. However, this method does require a sound knowledge of boolean algorithms plus a certain amount of practice. Another option for simplification will be introduced in the following section.

TP301 • Festo Didactic

B-30 Chapter 3

3.5 Karnaugh-Veitch diagram In the case of the Karnaugh-Veitch diagram (KV diagram) the truth table turns into a value table. Fig. B3.14: Truth table

a

b

c

d

y

No.

0

0

0

0

0

1

0

0

0

1

1

2

0

0

1

0

0

3

0

0

1

1

0

4

0

1

0

0

0

5

0

1

0

1

1

6

0

1

1

0

0

7

0

1

1

1

0

8

1

0

0

0

0

9

1

0

0

1

1

10

1

0

1

0

0

11

1

0

1

1

1

12

1

1

0

0

0

13

1

1

0

1

1

14

1

1

1

0

0

15

1

1

1

1

1

16

A total of 16 allocation options are available for the example, whereby the value table must also have 16 squares. Fig. B3.15: Value table

cd

cd

cd

cd

ab

1

2

3

4

ab

5

6

7

8

ab

9

10

11

12

ab

13

14

15

16

TP301 • Festo Didactic

B-31 Chapter 3

The results of the value table are transferred to the KV diagram according to the diagram shown below. In principle, representation is again possible in conjunctive or disjunctive standard form. The following, however, will be limited to the disjunctive standard form.

cd

cd

cd

cd

ab

0

1

0

0

ab

0

1

0

0

ab

0

1

0

1

ab

0

1

0

1

Fig. B3.16: Value table

The next step consists of combining the statuses, for which "1" has been entered in the value table. This is done in blocks whilst observing the following rules:


The combining statuses in the KV diagram must be in the form of a rectangle or square




The number of combining statuses must be a result of function 2x.

This results in the following:

cd

cd

cd

cd

ab

0

1

0

0

ab

0

1

0

0

ab

0

1

0

1

ab

0

1

0

1

Y1

TP301 • Festo Didactic

Y2

Fig. B3.17: Value table

B-32 Chapter 3

The variable values are selected for the established block and these in turn combined disjunctively. y1 = cd y2 = acd y = cd ∨ acd

(

)

= c ∨ ac ∧ d

(

)

= c∨a ∧d = cd ∨ ad Naturally, the KV diagram is not limited to 16 squares. 5 variables, for instance, would result in 32 squares (25), and 6 variables 64 fields (26).

TP301 • Festo Didactic

B-33 Chapter 4

Design and mode of operation of a PLC 4.1 Structure of a PLC With computer systems, differentiation is generally made between hardware, firmware and software. The same applies for a PLC, which is essentially based on a microcomputer. The hardware consists of the actual device technology, i.e. the printed circuit boards, integrated modules, wires, battery, housing, etc. Firmware is the software part, which is permanently installed and supplied by the PLC manufacturer. This includes fundamental system routines, used for starting the processor after the power has been switched on. Additionally, there is the operating system in the case of programmable logic controllers, which is generally stored in a ROM, a read-only memory, or in the EPROM. Finally, there is the software, which is the user program written by the PLC user. User programs are usually installed in the RAM, a random access memory, where they can be easily modified. Fig. B4.1: Fundamental design of a microcomputer

Data bus

Microprocessor (CPU)

ROM

RAM

Operatingsystem

Program and data

Address bus Control bus

TP301 • Festo Didactic

Inputmodule

Outputmodule

B-34 Chapter 4

Fig. B4.1 illustrates the fundamental design of a microcomputer. PLC hardware – as in the case of almost all of today’s microcomputer systems – is based on a bus system. A bus system is a number of electrical lines divided into address, data and control lines. The address line is used to select the address of a connected bus station and the data line to transmit the required information. The control lines are necessary to activate the correct bus station either as a transmitter or sender. The major bus stations connected to the bus system are the microprocessor and the memory. The memory can be divided into memory for the firmware and memory for the user program and data. Depending on the structure of the PLC, the input and output modules are connected to a single common bus or – with the help of a bus interface – to an external I/O bus. Particularly in the case of larger modular PLC systems, an external I/O bus would be more usual. Finally, a connection is required for a programming device or a PC, nowadays mostly in the form of a serial interface. Fig. B4.2 illustrates the Festo FPC 101 as an example. Fig. B4.2: Programmable logic controller Festo FPC 101

TP301 • Festo Didactic

B-35 Chapter 4

4.2 Central control unit of a PLC In essence, the central control unit of a PLC consists of a microcomputer. The operating system of the PLC manufacturer makes the universal computer into a PLC, optimised specifically for control technology tasks. Design of the central control unit Fig. B4.3 illustrates a simplified version of a microprocessor, which represents the heart of a microcomputer. Fig. B4.3: Design of a microprocessor

Data bus

ALU

Control bus

Accumulator

Command register

Control bus

Program counter

Arithmetic unit

Control unit Address bus

A microprocessor consists in the main of an arithmetic unit, control unit and a small number of internal memory units, so-called registers. The task of the arithmetic unit – the ALU (arithmetic logic unit) – is to execute arithmetic and logic operations with the data transmitted. The accumulator, AC for short, is a special register assigned directly to the ALU. This stores both data to be processed as well as the result of an operation. The instruction register stores a command called from the program memory until this is decoded and executed. A command consists of an operation part and an address part. The operation part indicates which logic operation is to be carried out. The address part defines the operands (input signals, flags etc.), with which a logic operation is to be executed.

TP301 • Festo Didactic

B-36 Chapter 4

The program counter is a register, which contains the address of the next command to be processed. The following section will be dealing with this in greater detail. The control unit regulates and controls the entire logic sequence of the operations required for the execution of a command. Instruction cycle within central control unit Today’s conventional microcomputer systems operate according to the so-called "von-Neumann principle". According to this principle, the computer processes the program line by line. In simple terms, you could say that each program line of the PLC user program is processed in sequence. This applies wholly irrespective of the programming language, in which the PLC program is written, be it in the form of a text program (statement list) or a graphic program (ladder diagram, sequential function chart). Since these various forms of representation always result in a series of program lines within the computer, they are subsequently processed one after the other. In principle, a program line, i.e. generally a command, is processed in two steps:


fetching the command from the program memory




executing the command

Fig. B4.4: Command sequence

Data bus Memory

Microprocessor Command Command register Control signals

Command

Programcounter +1 Addresses Address bus

TP301 • Festo Didactic

B-37 Chapter 4

The contents of the program counter are transferred to the address bus. The control unit then causes the command at a specified address in the program memory, to be relayed to the data bus. From there, the command is read to the instruction register. Once the command has been decoded, the control unit generates a sequence of control signals for execution. During the execution of a program, the commands are fetched in sequence. A mechanism, which permits this sequence, is therefore required. This task is performed by a simple incrementer, i.e. a step enabling facility in the program counter. 4.3 Function mode of a PLC Programs for conventional data processing are processed once only from top to bottom and then terminated. In contrast with this, the program of a PLC is continually processed cyclically. Fig. B4.5: Cyclical processing of a PLC program

Image table Inputs

Inputs

PLC program

Image table Outputs

TP301 • Festo Didactic

Outputs

B-38 Chapter 4

The characteristics of cyclical processing are:


As soon as the program has been executed once, it automatically jumps back to the beginning and processing is repeated.




Prior to first program line being processed, i.e. at the beginning of the cycle, the status of the inputs is stored in the image table. The process image is a separate memory area accessed during a cycle. The status of an input thus remains constant during a cycle even if it has physically changed




Similar to inputs, outputs are not immediately set or reset during a cycle, but the status stored intermediately in the process output image. Only at the end of a cycle are all the outputs physically switched according to the logic status stored in the memory.

The processing of a program line via the central control unit of a PLC takes time which, depending on PLC and operation can vary between a few microseconds and a few milliseconds. The time required by the PLC for a single execution of a program including the actualisation and output of the process image, is termed the cycle time. The longer the program is and the longer the respective PLC requires to process an individual program line, the longer the cycle. Realistic time periods for this are between approximately 1 and 100 milliseconds. The consequences of cyclical processing of a PLC program using a process image are as follows:


Input signals shorter than the cycle time may possibly not be recognised.




In some cases, there may be a delay of two cycle times between the occurrence of an input signal and the desired reaction of an output to this signal.




Since the commands are processed sequentially, the specific behaviour sequence of a PLC program may be crucial.

With some applications it is essential for inputs or outputs to be accessed directly during a cycle. This type of program processing, bypassing of the process image, is therefore also supported by some PLC systems.

TP301 • Festo Didactic

B-39 Chapter 4

4.4 Application program memory Programs specifically developed for particular applications require a program memory, from which these can be read cyclically by the central control unit. The requirements for such a program memory are relatively simple to formulate:


It should be as simple as possible to modify or to newly create and store the program with the help of a programming device or a PC




Safeguards should be in place to ensure that the program cannot be lost – either during power failure or through interference voltage




The program memory should be cost effective




The program memory should be sufficiently fast in order not to delay the operation of the central control unit.

Nowadays, three different types of memory are used in practice:


RAM




EPROM




EEPROM

RAM The RAM (random access memory) is a fast and highly cost effective memory. Since the main memory of computers (i.e. PLCs) consist of RAMs, they are produced in such high quantities that they are readily available at low cost without competition. RAMs are read/write memories and can be easily programmed and modified. The disadvantage of a RAM is that it is volatile, i.e. the program stored in the RAM is lost in the event of power failure. This is why RAMs are backed up by battery or accumulator. Since the service life and capacity of modern batteries are rated for several years, RAM back-up is relatively simple. Despite the fact that these are high performance batteries it is nevertheless essential to replace the batteries in good time.

TP301 • Festo Didactic

B-40 Chapter 4

EPROM The EPROM (erasable programmable read-only memory) is also a fast and low cost memory, which, in comparison with RAM, has the added advantage of being non-volatile, i.e. remanent. The memory contents therefore remain intact even in the event of power failure. Fig. B4.6: Example of an EPROM

For the purpose of a program modification, however, the entire memory must first be erased and, after a cooling period, completely reprogrammed. Erasing generally requires an erasing device, and a special programming unit is used for programming. Despite this relatively complex process of erasing, – cooling – reprogramming EPROMs are very frequently used in PLCs, since these represent reliable and cost effective memories. In practice, a RAM is often used during the programming and commissioning phase of a machine. On completion of the commissioning, the program is then transferred to an EPROM. EEPROM The EEPROM (electrically erasable programmable ROM), EEROM (electrically erasable ROM) and EAROM (electrically alterable ROM) or also flash-EPROM have been available for some time. The EEPROM in particular, is used widely as an application memory in PLCs. The EEPROM is an electrically erasable memory, which can be subsequently written to.

TP301 • Festo Didactic

B-41 Chapter 4

4.5 Input module The input module of a PLC is the module, which sensors are connected to. The sensor signals are to be passed on to the central control unit. The important functions of an input module (for the application) are as follows:


Reliable signal detection




Voltage adjustment of control voltage to logic voltage




Protection of sensitive electronics from external voltages




Screening of signals

Inputsignal

Error voltage detection

Signal delay

Optocoupler

Signal to the control unit

The main component of today’s input modules which meets these requirements is the optocoupler. The optocoupler transmits the sensor information with the help of light, thereby creating an electrical isolation between the control and logic circuits, thereby protecting the sensitive electronics from spurious external voltages. Nowadays, advanced optocouplers guarantee protection for up to approximately 5 KV, which is adequate for industrial applications. The adjustment of control and logic voltage, in the straightforward case of a 24 V control voltage, can be effected with the help of a breakdown diode/resistor circuit. In the case of 220 V AC, a rectifier is connected in series. Depending on PLC manufacturer reliable signal detection is ensured either by means of an additional downstream threshold detector or a corresponding range of breakdown diodes and optocouplers. Precise data regarding the signals to be detected is specified in DIN 19 240.

TP301 • Festo Didactic

Fig. B4.7: Block diagram of an input module

B-42 Chapter 4

The screening of the signal emitted by the sensor is critical in industrial automation. In industry, electrical lines are generally loaded heavily due to inductive interference voltages, which leads to a multitude of interference impulses on every signal line. Signal lines can be screened either via shielding, discrete cable ducts etc, or alternatively the input module of the PLC assumes the screening via an input signal delay. This therefore requires the input signal to be applied for a sufficiently long period, before it is even recognised as an input signal. Since, due to their inductive nature, interference impulses are primarily transient signals, a relatively short input signal delay of a few milliseconds is sufficient to filter out most of the interference impulses. Input signal delay is effected mainly via the hardware, i.e. via connection of the input to an RC module. In isolated cases, however, it is also possible to produce an adjustable signal delay via the software. The duration of an input signal delay is approximately 1 to 20 milliseconds – depending on manufacturer and type. Most manufacturers offer especially fast inputs for tasks, where the input signal delay is then too long to recognise the required signal. Differentiation is made between positive and negative switching connections when connecting sensors to PLC inputs. In other words, differentiation is made between inputs representing a current sink or a current source. In Germany for instance positive switching connections are mainly used, since this permits the use of protective grounding. Positive switching means that the PLC input represents a current sink. The sensor supplies the operating voltage or control voltage to the input in the form of a 1-signal. If protective grounding is employed, the output voltage of the sensor is short-circuited towards 0 volts or the fuse switched off in the event of a short-circuit in the signal line. This means that a logic 0 is applied at the input of the PLC.

TP301 • Festo Didactic

B-43 Chapter 4

In a number of countries, the use of negative switching sensors is commonplace, i.e. the PLC inputs operate as a power source. In these cases, a different protective measure must be used to prevent a 1-signal from being applied to the input of the PLC in the event of a shortcircuit on the signal line. Possible methods are the earthing of the positive control voltage or insulation monitoring, i.e. protective grounding as a protective measure. 4.6 Output module Output modules conduct the signals of the central control unit to final control elements, which are actuated according to the task. In the main, the function of an output – as seen from the application of the PLC – therefore includes the following:


Voltage adjustment of logic voltage to control voltage




Protection of sensitive electronics from spurious voltages from the controller




Power amplification sufficient for the actuation of major final control elements




Short-circuit and overload protection of output modules

In the case of output modules, two fundamentally different methods are available to achieve the above: Either the use of a relay or power electronics.

Signal from the control unit

Optocoupler Amplifier

Short-circuit monitoring

Output signal

The optocoupler once again forms the basis for power electronics and ensures the protection of the electronics and possibly also the voltage adjustment. A protective circuit consisting of diodes must protect the integral power transistor from voltage surges.

TP301 • Festo Didactic

Fig. B4.8: Block diagram of an output module

B-44 Chapter 4

Nowadays short-circuit protection, overload protection and power amplification are often ensured with fully integral modules. Standard short-circuit protection measures the current flow via a power resistor so as to switch off in the event of short-circuit; a temperature sensors provides overload protection; a Darlington stage or alternative power transistor stages provide the necessary power. The permissible power of an output module is usually specified in a way, which permits differentiation to be made between the permissible power of an output and the permissible cumulative power of an output module. The cumulative power of a module is almost always considerably lower than the total of individual permissible ratings, since power transistors transmit heat to one another. If relays are used for the outputs, then the relay can assume practically all the functions of an output module: The relay contact and relay coil are electrically isolated from one another; the relay represents an excellent power amplifier and is particularly overload-proof, only short-circuit protection must be ensured via an additional fuse. In practice, however, optocouplers are nevertheless connected in series with relays, since this renders the actuation of relays easier and simpler relays can be used. Relay outputs have the advantage that they can be used for different output voltages. By contrast, electronic outputs have considerably higher switching speeds and a longer service life than relays. In most cases, the power of the very small relays used in PLCs corresponds to that of the power stages of electronic outputs. In Germany for example, outputs are also connected positive switching, i.e. the output represents a power source and supplies the operating voltage to the consuming device. In the case of a short circuit of the output signal line to earth, the output is short-circuited, if normal protective grounding measures are used. The electronics switch to short circuit protection or the fuse switches off, i.e. the consuming device cannot draw any current and is therefore unconnected and rendered safe. (In accordance with EN 60204, the deenergised status must always be the safe status). If negative switching outputs are used, i.e. the output represents a current sink, the protective measure must be adapted in such a way, that the consuming device is rendered safe in the event of a short circuit on the signal line. Again, protective grounding with isolation monitoring or the neutralising of the positive control voltage are standard practice in this case.

TP301 • Festo Didactic

B-45 Chapter 4

4.7 Programming device/Personal computer Each PLC has a programming and diagnostic tool in support of the PLC application.


Programming




Testing




Commissioning




Fault finding




Program documentation




Program storage

These programming and diagnostic tools are either vendor specific programming devices or personal computers with corresponding software. Nowadays, the latter is almost exclusively the preferred variant, since the enormous capacity of modern PCs, combined with their comparatively low initial cost and high flexibility, represent crucial advantages. Also available and being developed are so-called hand-held programmers for mini control systems and for maintenance purposes. With the increasing use of laptop personal computers, i.e. portable, battery operated PCs, the importance of hand-held programmers is steadily decreasing. Essential software system functions forming part of the programming and diagnostic tool Any programming software conforming to EN 61131-1 (IEC 61131-1) should provide the user with a series of functions. Hence the programming software comprises software modules for:


Program input Creating and modifying programs in one of the programming languages via a PLC.




Syntax test Checking the input program and the input data for syntax accuracy, thus minimizing the input of faulty programs.




Translator Translating the input program into a program, which can be read and processed by the PC, i.e. the generation of the machine code of the corresponding PC.




Connection between PLC and PC This data circuit effects the loading of a program to the PLC and the execution of test functions.

TP301 • Festo Didactic

B-46 Chapter 4




Test functions Supporting the user during writing and fault elimination and checking the user program via – a status check of inputs and outputs, timers, counters etc. – testing of program sequences by means of single-step operations, STOP commands etc. – simulation by means of manual setting of inputs/outputs, setting constants etc.




Status display of control systems Output of information regarding machine, process and status of the PLC system – Status display of input and output signals – Display/recording of status changes in external signals and internal data – Monitoring of execution times – Real-time format of program execution




Documentation Drawing up a description of the PLC system and the user program. This consists of – Description of the hardware configuration – Printout of the user program with corresponding data and identifiers for signals and comments – Cross-reference list for all processed data such as inputs, outputs, timers etc. – Description of modifications




Archiving of user program Protection of the user program in nonvolatile memories such as EPROM etc.

TP301 • Festo Didactic

B-47 Chapter 5

Programming of a PLC 5.1 Systematic solution finding Control programs represent an important component of an automation system. Control programs must be systematically designed, well structured and fully documented in order to be as


error-free




low-maintenance




cost effective

as possible. Phase model of PLC software generation The procedure for the development of a software program illustrated in fig. B5.1 has been tried and tested. The division into defined sections leads to targeted, systematic operation and provides clearly set out results, which can be checked against the task. The phase model consisting of the following sections


Specification: Description of the task




Design: Description of the solution




Realisation: Implementation of the solution




Integration/commissioning: Incorporating into environment and testing the solution

can be applied to basically all technical projects. Differences occur in the methods and tools used in the individual phases.

TP301 • Festo Didactic

B-48 Chapter 5

Fig. B5.1: Phase model for the generation of PLC software

– Verbal description of control task – Technology, positional sketch – Macrostructure of control program

1.

Specification

2.

Design

– Function chart to IEC 60848 – Logic chart with symbols of the EN 60617-12 (IEC 60617) – Function table – Definition of software modules – Part list and circuit diagram

3.

Realisation

– Programming in LD, FBD, IL, ST and SFC – Simulation of subprograms and overall program

4. Commissioning

– Design of system – Testing of subprograms – Testing of overall program

The phase model can be applied to control programs of varying complexity; for complex control tasks the use of such a model is absolutely essential. The individual phases of the model are described below. Phase 1: Specification (Problem formulation) In this phase, a precise and detailed description of the control task is formulated. The specific description of the control system function, formalised as much as possible, reveals any conflicting requirements, misleading or incomplete specifications. The following are available at the end of this phase:


Verbal description of the control task




Structure/layout




Macrostructuring of the system or process and thus rough structuring of the solution

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Phase 2: Design (Concrete form of solution concept) A solution concept is developed on the basis of the definitions established in phase 1. The method used to describe the solution must provide both a graphic and process oriented description of the function and behaviour of the control system and be independent of the technical realisation. These requirements are fulfilled by the function chart (FCH) as defined in IEC 60848. Starting with a representation of the overall view of the controller (rough structure of the solution), the solution can be refined step by step until a level of description is obtained, which contains all the details of the solution (refinement of rough structure). In the case of complex control tasks, the solution is structured into individual software modules in parallel with this. These software modules implement the job steps of the control system. These can be special functions such as the realisation of an interface for visualisation or communications systems, or equally permanently recurring job steps. Phase 3: Realisation (Programming of solution concept) The translation of the solution concept into a control program is effected via the programming languages defined in EN 61131-3 (IEC 61131-3). These are: sequential function chart, function block diagram, ladder diagram, statement list and structured text. Control systems operating in a time/logic process and available in function chart to IEC 60848, can be clearly and easily programmed in a sequential function chart. A sequential function chart, in as far as possible, uses the same components for programming as those used for the description in the function chart to IEC 60848. Ladder diagram, function block diagram and statement list are the programming languages suitable for the formulation of basic operations and for control systems which can be described by simple operations logic operations or boolean signals. The high-level language structured text is mainly used to create software modules of mathematical content, such as modules for the description of control algorithms.

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In so far as PLC programming systems support this, the control programs or parts of a program created should be simulated prior to commissioning. This permits the detection and elimination of errors right at the initial stage. Phase 4: Commissioning (Construction and testing of the control task) This phase tests the interaction of the automation system and the connected plant. In the case of complex tasks, it is advisable to commission the system systematically, step by step. Faults, both in the system and in the control program, can be easily found and eliminated using this method. Documentation One important and crucial component of a system is documentation, which is an essential requirement for the maintenance and expansion of a system. Documentation, including the control programs, should be available both on paper and on a data storage medium. The documentation consists of the document of the individual phases, printouts of the control programs and of any possible additional descriptions concerning the control program. Individually these are:


Problem description




Positional sketch or technology pattern




Circuit diagram




Terminal diagram




Printouts of control programs in SFC, FBD etc..




Allocation list of inputs and outputs (this also forms part of the control program printouts)




Additional documentation

5.2 EN 61131-3 (IEC 61131-3) structuring resources EN 61131-3 (IEC 61131-3) is a standard for the programming of not just one individual PLC, but primarily also of complex automation systems. Control programs for extensive applications must be clearly structured in order to be intelligible, maintainable and possibly also portable, i.e. transferable to another PLC system.

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Definitions are required not only for elementary language commands, but also for the language elements for structuring. Structuring resources (fig. B5.2) relate to the control programs and the configuration of the automation system.

CONFIGURATION RESOURCE TASK VAR_GLOBAL ACCESS_PATH

Sequential function chart PROGRAM FUNCTION_BLOCK FUNCTION DATATYPES

– Configuration automation system

– Sequence representation – Refinement – Modularisation

Structuring of configuration level

Structuring of program level

Structuring resources at program level The structuring resources – program, function block and function – contain the actual control logic (rules) of the control program. These are also known as program organisation units. These structuring resources are available for any programming language. They are used for the modularization of control programs and the user program – this concerns primarily programs and function blocks – or also supplied by the manufacturer – as far as programs, function blocks are concerned. EN 61131-3 (IEC 61131-3) defines a comprehensive set of standardised functions and function block. These can be expanded by own user functions and function block for special or continually recurring tasks. Software modules, which can be used in any way, are entered in libraries, where they are made available. Programs represent the outer program organisation shell and can be differentiated from the function block mainly by the fact that they cannot be invoked by any other program organisation unit. The sequential function chart represents another resource for structuring at program level. The contents of the actual programs and function block can again be clearly and intelligibly represented by means of a sequential function chart.

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Fig. B5.2: EN 61131-3 (IEC 61131-3) structuring method

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Structuring resources at configuration level The language elements for configuration describe the incorporation of control programs in the automation system and their time-related control. The automation system represents a configuration (CONFIGURATION language element). Within the configuration, there are global variables VAR_GLOBAL language element). A resource (RESOURCE language element) corresponds to the processor of a multiprocessor system, to which one or several programs are assigned. In addition, it comprises control elements, which include the time-related control of programs. This control element is a task (TASK language element). The control element Task defines whether a program is to be processed cyclically or once only, triggered by a specific event. Programs not specifically linked to a task are processed cyclically in the background and with the lowest priority. Fig. B5.3: Graphic example of a configuration

Configuration Valve_production Resource Valve_assembly

Task_1

Resource Conveyor_control

Resource Quality_control Task_ cyclical

Task_2

Program Assembly

Program Conveyor

Program Initial_position_run

Program Conveyor_idle_run

Task_ unique

Program Packaging Program Statistics

Program Data_save Global and directly represented variables

The structuring resources for configuration are shown in a combined overview in fig. B5.3. An example applying these to an automation task is given by way of an explanation. The task in hand is to design and automate a production line for the assembly of pneumatic valves.

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A PLC multiprocessor with three processor cards has been designated for the valve assembly. The processor cards are assigned to the valve_assembly, the conveyor control and quality_control. The programs Statistics and Data_save are associated with different tasks. As such they possess different execution characteristics. The program Statistics evaluates and compresses the quality data at regular intervals. The priority of this program is low. It is started regularly, e.g. every 20 minutes, by the task Task_cyclical. In the event of an EMERGENCY- STOP, the program Data_save is to transmit all available data to a higher-order cell computer in order to prevent any potential data loss. The program is started event-driven of the highest priority via the EMERGENCY-STOP signal. EN 61131-3 (IEC 61131-3) provides defined and thus standardised interfaces for the exchange of data within a configuration. If specific information such as a read variable, is required in different program organisation units, this variable is designated as a global variable. Data can then be exchanged via a variable designated as such. Global variables can only be accessed in programs and function blocks. What is of interest for networked systems is communication beyond a configuration. Special standard communication function blocks are available to the user for this. These are defined in EN 61131-5 (IEC 61131-5) and are used in EN 61131-3 (IEC 61131-3). Another possibility is the definition of access paths (language resource ACCESS_PATH) to specific variables. These can then also be read or written from other positions.

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5.3 Programming languages EN 61131-3 (IEC 61131-3) defines five programming languages. Although the functionality and structure of these languages is very different, these are treated as one language family by EN 61131-3 (IEC 61131-3) with overlapping structure elements (variable declaration, organisation parts such as function and function block, etc.) and configuration elements. The languages can be mixed in any way within a PLC project. The unification and standardisation of these five languages represent a compromise of historical, regional and branch-specific requirements. Provision has been made for future expansion, (such as the function block principle or the language Structured Text) plus necessary information technology details (data type etc.) have been incorporated. The language elements are explained with the help of a machining process involved in valve production. Two sensors are used to establish whether a workpiece with correctly drilled holes is available at the machining position. If the valve to be machined is of type A or type B – this is set via two selector switches – the cylinder advances and presses the sleeve into the drilled hole. Ladder diagram (LD) Ladder diagram is a graphic programming language derived from the circuit diagram of directly wired relay controls. The ladder diagram contains contact rails to the left and the right of the diagram; these contact rails are connected to switching elements (normally open/normally closed contacts) via current paths and coil elements. Fig. B5.4: Example of ladder diagram language

Part_TypeA

Part_present

Drill_ok

Sleeve_in

Part_TypeB

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Function block diagram (FBD) In the function block diagram, the functions and function blocks are represented graphically and interconnected into networks. The function block diagram originates from the logic diagram for the design of electronic circuits. Fig. B5.5: Example of function block diagram language Part_TypeA

OR

AND

Sleeve_in

Part_TypeB Part_present Drill_ok

Instruction list (IL) Instruction list is a textual assembler-type language characterised by a simple machine model (processor with only one register). Instruction list is formulated from control instructions consisting of an operator and an operand. LD OR AND AND ST

Part_TypeA Part_TypeB Part_present Drill_ok Sleeve_in

With regard to language philosophy, the ladder diagram, the function block diagram and instruction list have been defined in the way they are used in today’s PLC technology. They are however limited to basic functions as far as their elements are concerned. This separates them essentially from the company dialects used today. The competitiveness of these languages is maintained due to the use of functions and function blocks.

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Fig. B5.6: Example of instruction list language

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Structured text (ST) Structured text is high-level language based on Pascal, which consists of expressions and instructions. Instructions can be defined in the main as: Selection instructions such as IF...THEN...ELSE etc., repetition instructions such as FOR, WHILE etc. and function block invocations. Fig. B5.7: Example of structured text language

Sleeve_in := (Part_TypeA OR Part_TypeB) AND Part_present AND Drill_ok;

Structured text enables the formulation of numerous applications, beyond pure function technology, such as algorithmic problems (high order control algorithms etc.) and data handling (data analysis, processing of complex data structures etc.). Sequential function chart (SFC) The sequential function chart is a language resource for the structuring of sequence-oriented control programs. The elements of the sequential function chart are steps, transitions, alternative and parallel branching. Each step represents a processing status of a control program, which is active or inactive. A step consists of actions which, identical to the transitions, are formulated in the EN 61131-3 (IEC 61131-3) languages. Actions themselves can again contain sequence structures. This feature permits the hierarchical structure of a control program. The sequential function chart is therefore an excellent tool for the design and structuring of control programs.

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Common elements of programming languages 6.1 Resources of a PLC According to EN 61131-3 (IEC 61131-3), only inputs and outputs and the controller memory can be addressed directly by a control program. Direct addressing in this instance means that in the program an input, output or memory element of the controller is affected immediately and not indirectly via a defined symbolic variable. Naturally, EN 61131-3 (IEC 61131-3) recognises numerous other resources, e.g. timers and counters. However, these are integrated into functions and function blocks in order to ensure the highest possible degree of control program portability between different control systems. Inputs, outputs and the memory The most important controller constituents include the inputs, outputs and the memory. Only via its inputs can a controller receive information from the connected processes. Similarly it can only influence these via its outputs or store information for subsequent continued processing. The designations for the resources inputs, outputs and memory elements are defined by EN 61131-3 (IEC 61131-3) and mandatory.

Inputs

I

Outputs

Q

Memory

M

Without further reference, these designate only binary inputs or outputs and single bit memory elements, designated as a flag. The standard generally speaks of directly represented variables. These are variables, which are referred directly to the hardware-related available inputs, outputs and memory elements of the controller. The allocation of inputs, outputs and flags and their physical or logical position in the control system is defined by the respective controller manufacturer.

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Fig. B6.1: Designations for inputs, outputs and memory

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Insofar as the controller supports this, resources can be addressed, which are defined in excess of one bit. EN 61131-3 (IEC 61131-3) employs a further letter to describe this, which follows the abbreviation I, Q or M and, for instance, designates bytes and words. EN 61131-3 (IEC 61131-3) designates the data types shown in fig. B6.2 in conjunction with inputs, outputs and flags. Fig. B6.2: Data types

BOOL

Bit sequence of length 1

BYTE

Bit sequence of length 8

WORD

Bit sequence of length 16

1-Bit sizes, such as defined by the data type BOOL (boolean), may only assume the values 0 or 1. Consequently, the range of values for BOOL type data consists of the two values 0 and 1. In contrast with this, one should observe that in the case of bit sequence data types consisting of more than one bit, there is no immediate connected range of values. All bit sequence data types such as for instance BYTE and WORD are merely a combination of several bits. Each of these bits has the value 0 or 1, but their combination does not have its own value. The mandatory designation methods for inputs, outputs and flags of different bit length are represented in fig. B6.3. Fig. B6.3 Designations for inputs, outputs and memory

I, Q, M or IX, QX, MX

Input bit, output bit, memory bit

1 Bit

IB, QB, MB

Input byte, output byte, memory byte

8 Bit

IW, QW, MW

Input word, output word, memory word

16 Bit

An individual bit of an input, output or flag may also be addressed without the additional abbreviation X for the data type.

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Since a controller always has a relatively large number of inputs, outputs and flags available, these must be specially identified for the purpose of differentiation. Numbering is used in EN 61131-3 (IEC 61131-3) to this end, such as in the following example:

I1

Input 1

IX9

Input 9

I15

Input 15

QW3

Output word3

MB5

Memory byte 5

MX2

Memory 2

EN 61131-3 (IEC 61131-3) does not specify the number range, which is permissible for this numbering and whether it should start with 0 or 1. This is specified by the controller manufacturer. A hierarchical number of inputs, outputs and flags may also be used, if the controller in use has been suitably configured. A point is used to separate the individual levels of the hierarchy. The number of hierarchy levels has not been defined. In the case of hierarchical numbering, the highest position in the number on the left must be coded, the numbers further to the right represent consecutive lower positions.


Example

I3.8.5

The specified input I3.8.5 can therefore be be made up as follows: Fig. B6.4: Structure of a hierarchical designation

Input in insert No. 3 on plug-in card No. 8 as input No. 5

I

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3.

8.

5

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EN 61131-3 (IEC 61131-3) does not make any comment regarding the assignment of individual bits in a BYTE or WORD. Controller manufacturers frequently choose hierarchical designation methods to assign individual bits as parts of words. As such, F6.2 could for instance represent the bit number 2 of flag word number 6. However, this does not necessarily have to be so, since flag bit F6.2 and flag word FW6 need not be in any way connected. Moreover, no definition has been made as to whether the numbering of individual bits in one word is to start on the left or the right (bit number 0 on the far right has been the most frequently used so far). Directly addressed variables If resources in a control program are to be addressed directly, the resource designation must be prefixed with the sign %. Examples of directly addressable variables:

%IX12 or %I12

Input bit 12

%IW5

Input word 5

%QB8

Output byte 8

%MW27

Memory word 27

The use of directly addressed variables is permissible solely in programs, configurations and resources. The program organisation units Function and Function block must operate exclusively with symbolic variables in order to keep these as controller- independent as possible and as such more widely usable. 6.2 Variables and data types The use of exclusively directly represented variables (resources, inputs, outputs and memory) is not enough to create control programs. Frequently, data is required, which contains specific information, also of a more complex nature. This data can be specified direct, e.g. time data or counter values or accessible via variables only – i.e. via a symbolic designation. The most important definitions for dealing with data or variables is shown below.

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Symbolic addressing A symbolic identifier always consists of capital or lower case letters, digits and an underline. An identifier must always begin with a letter or an underline. The underline can also be used to render an identifier more readable. It is however a significant character. The two identifiers Motor_on and Motoron are therefore different. Several underlines are impermissible. If the controller supports capital and lower case letters, then the use of these letters must not be of any significance. The two identifiers MOTORON and Motoron are interpreted identically and designate the same object. The following identifiers are impermissible:

123

Name does not start with a letter

Button_?

The last character is invalid, since it is neither a letter or a number

Furthermore, symbolic identifiers must not be identical with key words. As a rule, key words are words reserved for specific tasks. Representation of data Within a control program, it must be possible for time values, counter values etc. to be specified. Accordingly, EN 61131-3 (IEC 61131-3) has laid down the definitions for the representation of data to specify


Counter values




Time values




Strings

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Table B6.1: Representation of numerical data

Description

Examples

Integers

12, -8, 123_456*, +751

Floating point numbers

-12.0, -8.0, 0.123_4*

Numbers to base2 (Binary numbers)

2#1111_1111 2#1101_0011

(255 decimal) (211 decimal

Numbers to base8 (Octal numbers)

8#377 8#323

(255 decimal) (211 decimal

Numbers to base16 (Hexadecimal numbers)

16#FF or 16#ff 16#D3 or 16#d3

(255 decimal) (211 decimal

Boolean zero and one

0, 1

* The use of individual underlines between the digits is permissible to improve readability. However, the underline is not significant.

EN 61131-3 (IEC 61131-3) provides for different types of time data:

Table B6.2: Representation of time data




Duration, e. g. for measuring results




Date




Time of day, e. g. for synchronisation from start or end of an event (also in conjunction with date)

Description

Examples

Time duration

T#18ms, t#3m4s, t#3.5s t#6h_20m_8s TIME#18ms

Date

D#1994-07-21 DATE#1994-07-21

Time of day

TOD#13:18:42.55 TIME_OF_DAY#13:18:42.55

Date and time

DT#1994-07-21-13:18:42.55 DATE_AND_TIME#1994-07-21-13:18:42.55

The specification of a time duration consists of an introductory part, the key word T# or t#, and a sequence of time-related sections – i.e. days, hours, minutes, seconds and milliseconds.

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Abbreviations for time data: d

Day

h

Hour

m

Minute

s

Second

ms

Millisecond

Capitals may also be used instead of lower case letters and individual underlines inserted for the purpose of better readability. A fixed format has also been specified by EN 61131-3 (IEC 61131-3) for the specification of a date, time of day or a combination of both. Each specification starts with a key word; the actual information is represented as shown in table B6.2. Another important method of representation of data is the use of a sequence of characters also known as strings, which may be required for the exchange of information, e.g. between different controllers, with other components of an automation system or also for the programming of texts for display on control and display units. A string consists of zero or several characters, introduced and ended by a simple inverted comma. Example

Description

‘B’

String of length 1, containing the character B

‘Warning’

String of length 7, containing the string Warning

‘’

void string

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Table B6.3: Representation of strings

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Data types EN 61131-3 (IEC 61131-3) defines a large number of data types for different tasks. One such data type, BOOL, has already been mentioned. A BOOL type variable either assumes the value 0 or 1. Table B6.4 A number of elementary data types

Keyword

Data type

Range of values

BOOL

Boolean number

0, 1

SINT

Short integer

0 to 255

INT

Integer

-32 768 to +32 767

DINT

Double integer

-2 147 483 648 to +2 147 483 647

UINT

Unsigned integer

0 to 65 535

REAL

Floating point number

+/-2.9E-39 to +/-3.4E+38

TIME

Time duration

implementation-dependent

STRING

Variable-long string

implementation-dependent

BYTE

Bit sequence 8

no range of values declarable

WORD

Bit sequence 16

no range of values declarable

Two other important data types, named INT and UINT define integer numbers. Variables of data type INT (integer) permit numeric values of 32 768 to +32 767. The range of values of data type INT therefore covers both negative and positive numbers. Type UINT variables (unsigned integer) permit positive values only. The range of values for UINT extends from 0 to 65 535. SINT (short integer) and DINT (double integer) are additional data types defining integer numbers. However, these have an even smaller or greater range of values than data type INT. The data type REAL contains floating point numbers. These are numbers, which can contain places after the point, such as for instance 3.24 or -1.5. Data type TIME is used to specify time, and may contain a time duration such as for instance 2 minutes and 30 seconds. Apart from these elementary predefined data types, the user has the possibility of defining own data types. This is useful in cases where the problem definition goes beyond the realms of pure control technology. Derived data types are declared within a TYPE...END_TYPE construct. The complete declaration is listed below for enumeration type Colour in table B6.5.: TYPE Colour: (RED, BLUE, YELLOW, BLACK); END_TYPE

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Derived data type

Declaration TYPE … END_TYPE

Enumeration type

Colour: (RED, BLUE, YELLOW, BLACK);

Subrange type

Reference_range: INT(80..110);

Fields (array)

Position: ARRAY[1..10] OF REAL;

Structures

Coordinates: STRUCT X:REAL; Y:REAL; END_STRUCT;

The significance of the individual data types in table B6.5 is explained briefly below:


A data element of the type Colour may only assume one of the values RED, BLUE, YELLOW or BLACK.




A data element of the data type Reference_range may only assume values between 80 and 110, including the lower and upper limit 80 or 110.




A Position type data element represents a list with 10 entries. Each entry has the value of a REAL number. Individual entries can be indexed via the index.




A Coordinates type data element contains two REAL numbers, which can be accessed via their names X and Y.

Not every controller needs to recognise all these data types. Each controller manufacturer puts together a set of data types, which may be used in the controller. Variable declaration With the use of data, the right of access to this data must be clearly defined. To this end, EN 61131-3 (IEC 61131-3) uses a variable declaration. In order to understand the function of a variable declaration, it is first of all necessary to establish that the controller program is constructed into individual organisation units.

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Table B6.5: Derived data types

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These units are:


Configuration




Resource




Programs




Function blocks




Functions

All variables have a specific position. In the case of programming languages in text form (IL and ST), variable declarations are roughly the same as those used in the programming language Pascal. For graphic forms of representation, a tabular form with equivalent contents would be feasible. These are however not specified in EN 61131-3 (IEC 611313). All variable declarations (fig. B6.5) always start with a keyword, which designates the position of the variable in the organisation unit of the controller, and end with the keyword END_VAR. Fig. B6.5: Variable declaration

VAR Temp : INT; Hand : BOOL; Full, Open : BOOL; END_VAR

(*Temperature (*Flag for manual operation (*Flag for “full“ and “open”

*) *) *)

The variables and their assignment to a data type are entered between these keywords in that the symbolic identifier or identifiers of the variables are specified, the data type named after a colon and the declaration closed with a semicolon. If several variables are declared, they are repeated correspondingly. Normally, each declaration is written in a separate line in this case. EN 61131-3 (IEC 61131-3) differentiates between six different types of access to variables. Each type has a keyword, which introduces the variable declaration. Table B6.6: Keywords for the declaration of variables

Input variables

VAR_INPUT

Output variables

VAR_OUTPUT

In-/Output variables

VAR_IN_OUT

Local variables

VAR

Global variables

VAR_GLOBAL

External Variables

VAR_EXTERN

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Input variables are declared with the keywords VAR_INPUT and END_VAR. VAR_INPUT Input END_VAR

: INT;

(*Input value

*)

Fig. B6.6: Declaration of an Input variable

Variables specified in this way represent input variables fed externally to an organisation unit, e.g. a function block. These can be read only within the organisation unit. Modifications are not possible. Analogous to this, output variables are defined with the keywords VAR_OUTPUT and END_VAR. VAR_OUTPUT Result END_VAR

: INT;

(*Feedback value

*)

Fig. B6.7: Declaration of a Output variable

The data, which computes an organisation unit and feeds this back externally is declared as above. All organisation unit results are to be transferred beyond the organisation units via variables declared in this way. Within the organisation units, these can be read and written. Externally, read access only is permitted. In cases where variables containing input and output values are permitted, these must be created with the keywords VAR_IN_OUT and END_VAR. VAR_IN_OUT Value END_VAR

: INT;

This form represents a third option and permits the declaration of variables, which may be read and used within the organisation unit. In the case of a variable declared as VAR_IN_OUT, it is assumed that values will be supplied both to and from the organisation unit. Often, variables are required for intermediate results, which are to remain unknown externally. Locally named variables such as these are initiated with VAR and closed with END_VAR.

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Fig. B6.8: Declaration of a In-/Output variable

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Fig. B6.9: Declaration of a local variable

VAR Z END_VAR

: INT;

(*Intermediate result

*)

The variables specified here are local to an organisation unit and can only be used within this. They are unknown in all other organisation units and therefore inaccessible. One typical application are memory locations for intermediate results, which are not of any interest in other areas of the program. In the case of these variables, it should be noted that they may also exist several times in different organisation units. In this way, it is for instance possible for several function blocks to declare the local variable Z. These local variables are totally unrelated and differ from one another. A variable may also be globally declared, in which case it may be accessed universally. The necessary declaration is carried out in a similar way, whereby the keywords VAR_GLOBAL and VAR_EXTERNAL are used. Fig. B6.10: Declaration of a global variable

VAR_GLOBAL lobal value: INT; END_VAR

This is how all global data for a control program is declared. Global data is universally accessible. This declaration can only be found in the organisation units configuration and resource. Fig. B6.11: Declaration of access of a global variable

VAR_EXTERNAL Global value: END_VAR

INT;

In order to facilitate access of global data to an organisation unit, this declaration is to be recorded in the organisation unit. Without the declaration shown above, access to global data would not be permissible. This very strict declaration unit for all variables uniquely defines which variables are recognised by an organisation unit and how it may be used. A function block may for instance read but not change its input variables, and a program using a function block may read only but not change its output variables. The keyword AT is used to assign variables to the inputs and outputs of the controller.

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VAR Stop_button AT %12.3 Temperature AT %IW3 END_VAR

: :

BOOL; BOOL;

Fig. B6.12: Declaration of variables with assignment to inputs of a controller

Declarations in this form are the best means for defining the significance of all inputs and outputs of the controller. If a change occurs in the system and its connection to the controller, only these declarations need be changed. Any usage of the Stop_button, or the Temperature within an existing program, remain unaffected by this. According to EN 61131-3 (IEC 61131-3) it is however nevertheless possible to use directly addressed variables without being assigned to a symbolic identifier. The declaration in that case is as follows: VAR AT %14.2 AT %12.3 END_VAR

: BOOL; : WORD;

Initialisation Very often it is essential for a variable to be given an initial value. This value may change several times during the processing of a program, even though it is defined at the start. Initial statuses such as these are also important for other data. Such initial values are specified jointly with the declaration of the variables. A global variable of this type named Dozen is to be declared which, at the start of the program, assumes the value 12. VAR_GLOBAL Dutzend END_VAR

: INT :=12;

As shown by this example, the initialisation value is always inserted between the data type – in this instance INT – and the closing semicolon. The specification of the initialisation value always requires the prefixed symbol :=.

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Fig. B6.13: Declaration of a global variable with initial value

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In this way, each variable can be assigned a special initial value. Fundamentally, variables always have a defined initial value at the start of a program. This is facilitated by the characteristic defined in EN 61131-3 (IEC 61131-3), whereby data types already have a preset value. Each variable is preallocated the initial value of the corresponding data type – unless otherwise specified in the program. A list of initial values of a selection of elementary data types can be seen in table B6.7. Table B6.7: Preset initial values

Data type

Initial value

BOOL, SINT, INT, DINT

0

UINT

0

BYTE, WORD

0

REAL

0.0

TIME

T#0s

STRING

‘’ (void string)

6.3 Program The program for a controller is divided into individual organisation units, which are as follows at the programming level:


Programs




Function blocks




Functions

These program organisation units are available in all programming languages. EN 61131-3 (IEC 61131-3) defines a wide range of standardised functions and function blocks for typical control tasks. Apart from these specified functions and function blocks, EN 61131-3 (IEC 61131-3) permits the definition of own functions and function blocks. Manufacturers or users can thus produce tailor-made software modules for a particular application.

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Functions Functions are software modules which, when invoked provide exactly one result (data element). This is why in a text language the invocation of a function may be used as an operand in one expression. Functions cannot contain status information. This means that the invocation of a function with the same arguments (input parameters) must provide the same result. The addition of INT values or logic OR functions are examples for functions. Functions and their invocation may be represented graphically or in text form. Fig. B6.14: Graphic representation of a function

F Name X Y Z Inputs

Output

Graphically, a function is represented in the form of a rectangle. All input parameters are listed on the lefthand side, the output parameters are shown on the righthand side. The function name is entered within the rectangle. Formal input parameters may be specified along the edges within the rectangle. This is necessary with some function groups, such as the bit shift functions for instance (fig. B6.15b). For functions with identical inputs, such as in the case of the ADD function (fig. B6.15a), no formal parameter names are required.

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B-72 Chapter 6

Fig. B6.15: Use of formal parameters with functions

VAR AT AT AT AT

%QW4 %IW9 %IW7 %MW1

: : : :

INT; INT; INT; INT;

END_VAR

%QW4 %IW9 %IW7

%IW2 4

ADD

%MW1

a) without formal parameter names

SHL IN N

%MW5

b) with formal parameter names

The boolean inputs or outputs of a function may be negated, i.e. by specifying a circle directly outside the rectangle (fig. B6.16). Fig. B6.16: Representation of boolean negations %Q5.3

AND

%Q4.1

%M1.1

If a function is invoked, its inputs and the function output must be connected.

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B-73 Chapter 6

The ADD function illustrated in fig. 6.15a processes INT values, for which the deployed directly addressed variables such as %QW4 etc. are declared as variables of data type INT. Equally, the ADD function could be applied to type SINT or REAL counter values. Functions such as these, which operate to input parameters of a different data type are termed as overloaded, type-independent functions in EN 61131-3 (IEC 61131-3). Fig. B6.17 illustrates the characteristics of an overloaded function using the example of an ADD function. Fig. B6.17: Overloaded, typeindependent function

ADD function as example for an overloaded function All data types defining numbers are permissible as input and result parameters

ADD

a) Input parameters of type INT general

example VAR

INT INT

ADD

AT %IW1 AT %IW2 AT %MW3

INT

: INT; : INT; : INT;

END_VAR

%IW1 %IW2

ADD

%MW3

b) Input parameters of type SINT general

example VAR

SINT SINT

ADD

SINT

AT %IB4 AT %IB5 AT %MB6

: SINT; : SINT; : SINT;

END_VAR

%IB4 %IB5

TP301 • Festo Didactic

ADD

%MB6

B-74 Chapter 6

If an overloaded function is limited to a particular data type by the controller – i.e. data type INT as shown in fig. B6.18 – this is referred to as a typed function. Typed functions are recognisable by their function name. Typing is indicated by the addition of the underline, followed by the desired type. Fig. B6.18: A typed function example

general

VAR INT

ADD_INT

INT

INT

AT %IW1 AT %IW2 AT %MW3

: INT; : INT; : INT;

END_VAR

%IW1

ADD_INT

%MW3

%IW2

Standard functions The most important standard functions for the realisation of basic control technology tasks are listed below. Since a wide variety of standard function are able to operate using input parameters of different data types, the data types have been combined into groups. Each group is given a generic data type. The most important generic data types are shown in table B6.8. Table B6.8: Generic data types

ANY_NUM

all data types for floating point numbers such as REAL and for integer numbers such as INT, UINT etc., are contained in ANY_REAL and ANY_INT

ANY_INT

all data types for integers such as INT, UINT etc.

ANY_REAL

all data types, defining floating-point numbers such as REAL or LREAL

ANY_BIT

all bit sequence data types such as BOOL, BYTE, WORD etc.

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B-75 Chapter 6

***

ANY_BIT

ANY_BIT

ANY_BIT ...

Table B6.9: Bit-by-bit boolean functions

... ANY_BIT * * * = name or symbol

Name

Symbol

AND

&

OR XOR

Description AND operation of all inputs

>=1

OR operation of all inputs

=2k+1

Exclusive OR operation of all inputs

NOT

Negating input Table B6.10: Bit shift functions

*** ANY_BIT

IN

ANY_INT

N

ANY_BIT

* * * = name Name

Description

SHL

Shift IN by N bits to the left, fill with zeros to the right

SHR

Shift IN by N bits to the right, fill with zeros to the left

ROR

Shift IN cyclically by N bits to the right

ROL

Shift IN cyclically by N bits to the left

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B-76 Chapter 6

Table B6.11: Comparison functions

ANY_BIT or ANY_NUM ...

ANY_BIT or ANY_NUM

***

...

* * * = name or symbol

Name

Table B6.12: Functions for type conversion

Symbol

Description

GT

>

Greater (falling sequence)

GE

>=

Greater or equal (monotonic sequence)

EQ

=

Equal

LE

=1

Gate_closed

>

Temp 60

b) Use of function blocks T_Startup TON %I 1.3

IN

Q

T#7s

PT

ET

%Q2.4

The elements of the function block diagram are graphically represented functions and function blocks. These are interconnected by signal flow lines; directly linked elements form a network. Fig. B7.1 illustrates two simple examples of the function block diagram. In fig. B7.1a, the variable Manual_off and the result Greater-Comparison are OR’d. The result is mapped onto the variable Gate_closed. Fig. B7.1b represents the use of a function block. The signal delay T_Startup is started via the input %I1.3 with the preset time of 7 seconds. The signal delay status, T_Startup.Q, is assigned to the output %Q2.4.

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B-86 Chapter 7

7.2 Evaluation of networks The direction of the signal flow in a network is from left to right. If a program organisation unit consists of several networks, these are processed in sequence from top to bottom. The processing sequence within a program organisation unit may be influenced through the use of elements for execution control. This group of elements includes for instance the conditional and the unconditional jump. In fig. B7.2 a conditional jump is used to realise a program branch. Fig. B7.2: Use of a jump in FBD

%I 1.1

&

Variant_1

%M2.5

Variant_1: %M2.1 %I 2.5

>=1

%Q1.0

The conditional jump, represented via a double arrow, is executed, if the jump condition is met. Therefore, if input %I1.1 and flag %M2.5 both carry a 1-signal, then a jump will be executed to network with the identifier Variant_1 and processing continued at this point. If a jump is to be executed to a network, the corresponding network must be prefixed with a symbolic name, the jump flag, ending with a colon. The jump flag must be identified in accordance with the rules for symbolic names.

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B-87 Chapter 7

7.3 Loop structures When programming in the FBD language, it should be noted that no loop structures are permissible within networks. Structures of this type may be realised solely through the additional use of a feedback path. Fig. B7.3b illustrates an example of this. Fig. B7.3: FBD with loop structures

a) unauthorised loop structure %M4 %I 1

&

>=1

%I 2

b) authorised loop structure %M4 %I 1

&

%M2.0

>=1

%I 2

By means of the utilisation of a feedback path, the third input of the ORfunction assumes a defined value during its processing.

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TP301 • Festo Didactic

B-89 Chapter 8

Ladder diagram 8.1 Elements of the ladder diagram The ladder diagram language, like the function block diagram, represents a graphic programming language. The elements available in a ladder diagram are contacts and coils in different forms. These are laid out in rungs within the confines of power rails on the left and on the right.

%M1.5

%Q3.5

Fig. B8.1 illustrates the basic structure of a current rung. In this example, the status of the flag %M1.5 is directly assigned to %Q3.5. Table B8.1 contains a list of the most important elements of a ladder diagram.

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Fig. B8.1: Basic structure of a rung

B-90 Chapter 8

Table B8.1: Elements of the ladder diagram

Contacts Normally open contact /

Normally closed contact

P

Edge contact, positive edge

N

Edge contact, negative edge

Coils Coil /

Negating coil

S

Setting coil

R

Resetting coil

P

Edge coil, positive edge

N

Edge coil, negative edge

A normally open contact supplies the value 1, when the corresponding push button switch is closed. A normally closed contact reacts correspondingly with the value 1, when the switch or push button is opened. Two edge contacts supply the value 1 for the transition from 0 to 1 (positive edge) or from 1 to 0 (negative edge). With a normal coil, the result (resulting logic operation of the contacts) is copied to the specified variable; in the case of a negating coil, the negation of the result is transferred. The setting coil assumes the value 1, if the result is 1, and remains unchanged even if the result is 0 in between. Similarly the resetting coil only assumes the value 0, if the result is 1. The 0 status of the coil is maintained. The two edge coils are set, if the result changes from 0 to 1 (positive edge) or from 1 to 0 (negative edge. The basic functions AND and OR may be realised by means of a corresponding configuration of contacts in the current rung.

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B-91 Chapter 8

Fig. B8.2 Basic logic connections in ladder diagram

a) AND function %I 1.3

%M3.2

%Q2.1

b) OR function %I 1.5

%Q2.3

%M3.4

The AND function is represented by means of the serial connection of the two contacts (fig. B8.2a). Output %Q2.1 is set only if both input %I1.3 and flag %M3.2 are set. In all other cases, output %Q2.1 is reset. The OR function is obtained via parallel switching of contacts (fig. B8.2b). Output %Q2.3 assumes the value 1, if either input %I1.5 or flag %M3.4 assume the value 1, or if both conditions are met simultaneously.

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B-92 Chapter 8

8.2 Functions and function blocks Apart from the contact and coil elements, LD provides the unlimited use functions and function blocks in so far as this feature is supported by the controller used. Prerequisite for the incorporation of so-called organisation units, is the availability of at least one boolean input and one boolean output of the block in question. If this is not the case, a boolean input with the formal parameter EN (enable) is added to the corresponding functions or function modules as well as a boolean output ENO (enable OK). The boolean inputs/outputs are required to permit the power flow through the block. Fig. B8.3: Functions and function blocks in ladder diagram

a) Incorporation of functions Add_akt

Add_ok

+

EN ENO Quantity_1 Quantity_2

Filling_level

b) Invocation of function block T_Startup %I 1.3

TON T#7s

IN

Q

PT

ET

%Q2.4

The addition shown in fig. B8.3a is only undertaken, if a 1-signal is applied at the input EN. If this is the case, the variables Quantity_1 and Quantity_2 are added and the result of these variables assigned to the variable Filling_Level. At the same time, the value of output ENO indicates, whether the addition has been executed, activated and correct (ENO=1). If the block has not been processed correctly, the output ENO assumes the value 0.

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B-93 Chapter 8

Function modules such as for instance the signal delay shown in fig. B8.3b can be incorporated in the ladder diagram without additional EN input and ENO output. The function block is connected with the elements of the current rung in the usual manner via the boolean input IN and the boolean output Q. If input %I1.3 in fig. B8.3b assumes the value 1, the function block copy T_Startup is processed with the preset time duration of 7 seconds. The value of output Q of T_Startup is assigned to output %Q2.4. 8.3 Evaluation of current rungs Similar to the graphic programming language FBD, the power flow, and as such processing within a program organisation unit, is from left to right and from top to bottom. Equally, the processing sequence may also be changed in LD by using elements for execution control. %I 1.1

%M2.5

Fig. B8.4: Conditional jump in LD

Variant_1

Variant_1: %M2.1

%Q1.0

%I 2.5

If the jump condition, in this case the AND operation of input %I1.1 and flag %M2.5, is met, a jump is executed to the current rung with the identifier Variant_1. Processing is then continued from this current rung onwards.

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B-94 Chapter 8

TP301 • Festo Didactic

B-95 Chapter 9

Instruction list 9.1 Instructions Instruction list is a textual, assembler-type programming language. Its instructions most closely reassemble the commands processed in a PLC. A control program formulated in the Instruction List language consists of a series of instructions, whereby each instruction must begin with a new line. A fixed format is specified for the formulation of an instruction. An instruction (fig. B9.1) starts with an operator with optional modifier and, if necessary for the particular operation, one or several Operands, separated by commas. Instructions may be preceded by a label followed by a colon. The label acts as a jump address. Labels are identified in the same way as symbols. If a comment is used, this must represent the last element of the line. A comment is introduced via the string (*, and ended by the string *). Fig. B9.1: Structure of an instruction Instruction

Label

Start:

Operator and modifier

LD AND ST

Operand

%I1.2 %M3.7 %Q2.4

Comment

(* (* (* (*

Part present Drill ok Stamp advance

*) *) *) *)

The value of input %I1.2 is loaded to the accumulator and ANDed with value of flag %M3.7. The resultant actual result is assigned to output %Q2.4.

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B-96 Chapter 9

9.2 Operators

Table B9.1: Instruction list operators

Operator

Modifier

Operand

Description/Significance

LD

N

Reads the specified operand to the accumulator and equates the current result to this operand

ST

N

Stores the current results at specified operands

S

BOOL

Sets boolean operand to the value 1, if the value of the current result is a boolean 1

R

BOOL

Resets the boolean operand to 0, if the value of the current result is a boolean 1

AND

N, (

BOOL

Boolean AND

&

N, (

BOOL

Boolean AND

OR

N, (

BOOL

Boolean OR

XOR

N, (

BOOL

Boolean exclusive OR

ADD

(

Addition

SUB

(

Subtraction

MUL

(

Multiplication

DIV

(

Division

GT

(

Comparison: >

GE

(

Comparison: >=

EQ

(

Comparison: =

NE

(

Comparison: < >

LE

(

Comparison: =1

%IX0.5 %MX1

%QX1.2

&

%MX3

Fig. B11.15: Formulation of actions: graphic declaration in LD language

Action_2 %IX0.5 %MX1

Fig. B11.16: Formulation of actions:: textual declaration

IL language ACTION Action_2: LD %IX0.5 OR( %MX1 AND %MX3 ) ST %QX1.2 END_ACTION

%QX1.2 %MX3

ST langugae ACTION Action_2: %QX1.2 := %IX0.5 OR (%MX1 AND %MX3); END_ACTION

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B-127 Chapter 11

Instead of an individual network of a statement sequence, several networks are also permissible within an action in textual languages. In this way, it is possible to incorporate a very wide range of actions in a step, and again a step itself may contain sequence structures (fig. B11.17). Fig. B11.17: Formulation of actions: Inclusion of SFC elements in an action

Action_4

Start Part_present Color C_ok

L T#1s

Color_determine

If individual fields of an action block are not required, such as for instance if a boolean variable is used as action content, a further simplification in the representation of an action block is permissible.

S

%QX12

A feedback variable (c field) may be entered in each action block. Feedback variables are programmed within actions by the user and indicate their completion, timeout or error conditions. Fig. B11.19 indicates a frequently recurring application. Here the sequence of steps and transitions is structured in such a way that the action of a step sets the subsequent step enabling condition.

TP301 • Festo Didactic

Fig. B11.18: Short representation of an action block

B-128 Chapter 11

Fig. B11.19: Use of feedback variables

Step_2

S

Cylinder_1

Pos_1

S S

Cylinder_2 Vacuum_on

Vac_on

R

Cylinder_1

Pos_2

Pos_1 Step_3 Vac_on Step_4 Pos_2

Mode of action of action qualifiers The type of execution of the actions programmed by the user is defined by the associated action qualifier. EN 61131-3 (IEC 61131-3) defines the following action qualifiers


N

Non-stored




S

Set (stored)




R

Overriding reset




P

Pulse (unique)




L

Time limited




D

Time delayed




DS Time delayed and stored




SD Stored and time delayed




SL Stored and time limited

Each action is the equivalent of exactly one of these qualifiers. In addition, the qualifiers L, D, DS, SD, SL have an associated duration of type Time, since these define a time behaviour of the action.

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B-129 Chapter 11

The qualifiers have a precisely defined significance. If a step is inactive, none of the action of the step is executed. With an active step, the following methods apply for the execution of an action qualifier. N Non-stored


the action is executed for as long as the step is active. Fig. B11.20: Non-stored action N Step active %QX12

%QX12

1 0 1 0

In the above example, the output %QX12 is set for as long as the step containing this action is active. On completion of the step, i.e. as soon as the subsequent enabling condition is met, the output is automatically reset. S Set (stored)


the execution of the action is executed permanently set (set stored). Fig. B11.21: Set (stored) action S Step active %QX12

%QX12

1 0 1 0

In this example, the output %QX12 is set for as long as the step containing this action is active. The output also remains set, when the subsequent step enabling condition is met and the step being considered is no longer active. The output can only be reset in another step, in another action, defined with qualifier R.

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B-130 Chapter 11

R Reset


a previously set action (in another step) executed with the qualifier S, DS, SD, L or SL is cancelled.

Fig. B11.22: Reset action R Step active %QX12

%QX12

1 0 1 0

Output %QX12 has been set in another step in an action with one of the qualifiers S, DS, SD, L or SL and reset again via this action. P Pulse (unique)


unique execution of the action.

Fig. B11.23: Unique action P Step active %QX12

%QX12

1 0 1 0

During the initial execution of the action via the controller within the processing of the step, output %QX12 is set exactly once and then reset again. The output is reset uniquely again only after the exiting the step and a fresh re-entry into the step.

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B-131 Chapter 11

L Time limited


the action is executed for a specific time. Fig. B11.24: Time limited action L T#10s

Step active %QX12

%QX12

1 0 1 0

10s

10s

Output %QX12 is set for 10 seconds and subsequently reset again. This requires the step containing this action to be active for a period of at least 10 seconds. If the subsequent step enabling conditions are met prior to this time, the action time of the output is reduced also, since it is reset at the end of the step in any case. D Time delayed


the execution of the action is delayed until the end of the step. Fig. B11.25: Time delayed action D T#10s

Step active %QX12

%QX12

1 0 1 0

10s

10s

Here, output %QX12 is not set until 10 seconds have expired and remains set until the step becomes inactive. If the duration during which the step is activated is less than 10 seconds, the output will not be set during the processing of this step.

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B-132 Chapter 11

DS Time delayed and stored


the execution of the action is time-delayed and maintained beyond the end of the step.

Fig. B11.26: Time delayed and stored Action

DS T#10s Step active

%QX12

%QX12

1 0

10s

10s

1 0

1 R active 0 (in another step)

In this example too, output %QX12 is set after 10 seconds have expired. However, it remains set after completion of the step. It must be reset explicitly via another action with the qualifier R (in another step). If the duration of the step is not sufficiently long, in this case less than 10 seconds long, the output will never be set.

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B-133 Chapter 11

SD Stored and time delayed


the execution of the action is time delayed and is maintained beyond the end of the step.

SD T#10s

Fig. B11.27: Stored and time delayed action

%QX12

1

Step active

0

10s

10s

1

%QX12

0

1 R 0 active (in another step)

Here too, output %QX12 is set after the expiry of 10 seconds. It remains set following the end of the step and can only be explicitly reset via another action with the qualifier R in another step. Unlike the mode of action of the DS qualifier, it is not necessary for the step to remain active beyond the duration of the delay for the output to be set. SL Stored and time-limited


the action is executed continuously for a specific period.

SL T#10s Step active

%QX12

Fig. B11.28: Stored and time limited action

%QX12

1 0 1 0

TP301 • Festo Didactic

10s

10s

B-134 Chapter 11

The output is set for 10 seconds and then reset again. In contrast with the mode of action of the L qualifier, it is not necessary for the step to be active for a minimum of 10 seconds. If the subsequent step enabling condition is met prior to this time expiring, i.e. if the step is active for less than 10 seconds, the active period of the output remains unaffected by this. The output can be reset at any time via another action with the qualifier R. The mode of action of the individual qualifiers has been illustrated in the example of a boolean variable as an action. If more complex, i.e. non boolean actions are required, the type of execution is marginally different to the previously examined boolean variables. The networks are continually processed for as long as the step is active. As soon as the subsequent step enabling condition is met, however, the last, unique, execution of the networks is carried out once more. This definition enables the targeted resetting of variables at the end of the processing of an action, when the N qualifier is used for more complex actions. Fig. B11.29: Complex action in FBD language

Step_5

N

Action_1 &

%IX1.0

%QX1.0

%MX12 Step_5.X

SR_1 SR %IX1.5

S1

Q1

%QX1.5

R

If step Step_5 is deactivated, the last processing of the networks takes place with the value 0 for step flag Step_5.X. This results in output %QX1.0 being reset to 0 when the step is exited.

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B-135 Chapter 11

11.5 Example Problem description Components are transported together on a conveyor belt to a dual processing station. The drilling and countersinking units then move downwards as soon as a component is present. Two cylinders 1A1 and 2A1 are used to move the two machine tools. The conveying device is indexed by one working position via a third cylinder 3A1. Two sensors B4 and B5 are provided to detect whether a workpiece is located underneath the drill or the countersink. The required drilling and countersink depths are sensed via two end position sensors 1B2 and 2B2. The initial positions of the transport cylinder, drill and countersink can be detected via the values of sensors 1B1, 2B1 and 3B1. Sensor 3B2 indicates an extended transport cylinder. The system cannot always guarantee that a workpiece will be deposited underneath both the drill unit and the countersink after each transport movement. Processing should then be interrupted in the case of a missing workpiece. If both workpieces are missing at the same time, neither of the two tools should be lowered.

1A1

Fig. B11.30: Positional sketch

2A1

1B1

2B1

1B2

2B2

B4

B5

3A1 3B1

TP301 • Festo Didactic

3B2

B-136 Chapter 11

Allocation list Table B11.4: Allocation list

Equipment designation

PLC input/ PLC output

Task

B4

%IX0.1

Detecting workpiece under the drill

B5

%IX0.2

Detecting workpiece under countersink

1B1

%IX0.3

Initial position of drill unit (up)

2B1

%IX0.4

Initial position of countersink (up)

3B1

%IX0.5

Initial position of transport cylinder

1B2

%IX0.6

Lower end position of drill unit reached

2B2

%IX0.7

Lower end position of countersink reached

3B2

%IX0.8

Transport cylinder extended

1Y1

%QX0.1

Lower drill unit

2Y1

%QX0.2

Lower countersink

3Y1

%QX0.3

Transport feed

Problem A control program is to be designed for this task. The solution is to achieve a configuration by means of a sequential function chart. The conditions and actions are then to be applied to the steps and transitions. The program is to be executed cyclically. To simplify matters, you may assume that there is no need to use timers to compensate for positioning tolerances.

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B-137 Chapter 11

Solution

Fig. B11.31: Program in sequence language

Start 1B1

2B1

3B1

B4

R 1Y1 R 2Y1 R 3Y1

B4

B5

/ Drill

S 1Y1

1B2

R 1Y1

1B1

Countersink

C_up 2B1

1 Transport

TP301 • Festo Didactic

/ S 2Y1

2B2

D_up

3B2

B5

S 3Y1

R 2Y1

B-138 Chapter 11

All cylinders are brought into their initial position in one initial step. At the end of the program, this step is also used to retract the cylinder extended during the last step for the transport device. When all the cylinders are in their initial position, a parallel branch with two sequences is started for drilling and countersinking. Both sequences in this example contain the same tasks, but use different tools. The lefthand sequence lowers the drill unit and lifts it again, and the righthand sequence evolves identically for the countersinking. The sequences merely differ in their use of sensors and actuators. A void step is incorporated at the top and bottom of both sequences to maintain the necessary sequence steps and transitions. The program for the drilling unit evolves as follows. It detects whether a workpiece is available via the value of sensor B4. If this value is equal to 1, the workpiece is in the required position and the drilling process starts. Otherwise the entire drilling process is bypassed in an alternative path. Drilling of the hole starts with the lowering of the drill by setting 1Y1. When the lower end position is reached, i.e. drilling of the hole has been completed, sensor 1B2 signals the end of drilling. In the next step, the drill is returned to its normal position at the top. This part of alternative branching ends when the drill has reached the top. The program follows the same procedure for countersinking. When both parallel sequences have been completed, a transition takes place in the program to the transport step. The necessary synchronisation – i.e. drilling and countersinking ready – is ensured by the sequential function chart and therefore does not require special treatment. A true step enabling condition is always inserted at this point in order for the steps and transitions to alternate. In the last step Transport, the cylinder of the transport device is extended and the awaited completed action in the next transition condition. Thereafter, the whole process starts anew.

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B-139 Chapter 12

Logic control system 12.1 What is a logic control system Logic control systems are controllers programmed through the use of boolean operations. All logic operations are processed and executed during a machining cycle. Control tasks realised typically in the form of logic control, are characterised by the fact that no time duration is given within the process, but all or most of the conditions of the control program are examined simultaneously. Examples of logic control systems can therefore be found in PLC applications, where safety aspects are of importance. The monitoring of certain tasks is often required to be permanent and independent of the time-related execution of the process. These requirements apply for instance in:


Protective circuits: a device may only load, if all protective devices are switched on




Safety interlocking

12.2 Logic control systems without latching properties Logic control systems without latching properties may be described by means of a combination of boolean operations, whereby the output signals of a controller are determined by a combination of input signals at a given time. The basic logic operations AND, OR and NOT may be used to create any complex logic operations – and as such also control systems. A number of boolean algebra methods such as function tables, Boolean equations and disjunctive normal form (DNF) are used to describe the problem and to find a solution. The importance of these methods is demonstrated amongst other things in the more complex applications for logic control systems. The actual programming of a logic control system is preferably in the languages ladder diagram or function block diagram.

TP301 • Festo Didactic

B-140 Chapter 12

Typical boolean operations The following represents basic control technology tasks such as boolean operations realised via PLC. The solutions are represented in the languages LD, FBD, IL and ST. The solution sections are preceded by a declaration of the necessary PLC inputs and outputs. In addition, the description options of a function table and boolean equation are also listed. Negation The output signal assumes the value 1, if the input signal has the value 0 and vice versa. Example

Lamp H1 is illuminated as long as switch S1 is not actuated; it is extinguished, if the switch is closed. The purpose of S1 is therefore to switch off the lamp.

Fig. B12.1: Description methods

Fig. B12.2: Declaration of variables

Function table

Boolean equation

S1 H1

H1 = S1

0

1

1

0

VAR

S1 AT %I2.5: BOOL; H1 AT %Q1.4: BOOL; END_VAR

TP301 • Festo Didactic

B-141 Chapter 12

a) LD

b) FBD S1

H1

/

NOT

S1

c) IL LDN ST

Fig. B12.3: Negation

H1

d) ST S1 H1

H1

:=

NOT S1;

AND-operation The output signal only assumes the value 1, if all input signals have the value 1. Lamp H1 is to be switched on only if the two switches S1 ad S2 are actuated.

Boolean equation

S1 S2 H1

H1 = S1 S2

0

0

0

0

1

0

1

0

0

1

1

1

VAR

S1 AT %I2.5: BOOL; S2 AT %I2.6: BOOL; H1 AT %Q1.4: BOOL; END_VAR

TP301 • Festo Didactic

Fig. B12.4: Description methods

<

Function table

Example

Fig. B12.5: Declaration of variables

B-142 Chapter 12

Fig. B12.6: AND operation

a) LD

b) FBD S1

S2

H1

&

S1

H1

S2

c) IL

d) ST

LD AND ST

S1 S2 H1

H1

:=

S1 AND S2;

OR-operation The output signal assumes the value 1, if at least one input signal has the value 1. Lamp H1 is to be switched on, if at least one switch, S1 or S2 is actuated.

Fig. B12.7: Description methods

Fig. B12.8: Declaration of variables

Function table

Boolean equation

S1 S2 H1

H1 = S1 S2

0

0

0

0

1

1

1

0

1

1

1

1

<

Example

VAR S1 AT %I2.5: BOOL; S2 AT %I2.6: BOOL; H1 AT %Q1.4: BOOL; END_VAR

TP301 • Festo Didactic

B-143 Chapter 12

a) LD

Fig. B12.9: OR operation

b) FBD S1

H1

>=1

S1

H1

S2 S2

c) IL

d) ST

LD OR ST

S1 S2 H1

H1

:=

S1 OR S2;

Combined logic operations Lamp H1 is to be illuminated only, if precisely two of the three switches S1, S2, S3 are actuated.

Example

The first to be created is the function table, whereby those combinations are selected, which provide the result 1. These are lines 4, 6 and 7. The boolean equation and thus the solution can be created on the basis of this combination. The conversion of the solution into the individual programming languages is contained in fig. B12.12.

Function table

Boolean equation

S1 S2 S3 H1

H1 = (S1 S2 S3) (S1 S2 S3) (S1 S2 S3)

0

0

0

0

0

0

1

0

0

1

0

0

0

1

1

1

1

0

0

0

1

0

1

1

1

1

0

1

1

1

1

0

TP301 • Festo Didactic

Fig. B12.10: Description methods

B-144 Chapter 12

Fig. B12.11: Declaration of variables

Fig. B12.12: Combination of boolean operations

VAR S1 AT S2 AT S3 AT H1 AT END_VAR

%I2.5: BOOL; %I2.6: BOOL; %I2.7: BOOL; %Q1.4: BOOL;

a) LD

b) FBD S1

S2

S3

S2

S3

H1

/ S1

/ S1

S2

S3 /

c) IL LD ( AND ANDN ) OR ( ANDN AND ) OR ( AND ANDN ) ST

S1 S2 S3

&

S1 S2 S3

&

S1 S2 S3

&

>=1

H1

d) ST S3 S2 S1

H1

:= (NOT S1 AND S2 AND S3) OR (S1 AND NOT S2 AND S3) OR (S1 AND S2 AND NOT S3);

S1 S2 S3 S1 S2 S3 H1

TP301 • Festo Didactic

B-145 Chapter 12

12.3 Logic control systems with memory function Many PLC applications require elementary memory operations. A memory function constitutes the retention, i.e. storage of a briefly occurring signal status. At a given instant, the output signals are dependent not only on the combination of input signals, but also on "previous statuses". The example given here is that of a switch for switching on and off a lamp. EN 61131-3 (IEC 61131-3) defines two function blocks for the realisation of memory functions. These are function block SR (primarily setting) and RS (primarily resetting). A description of the blocks follows below. Function block SR Fig. B12.13: Function block SR, primarily setting

SR BOOL

S1

BOOL

R

Q1

BOOL

The standard function block SR contains a dominant setting flipflop (bistable memory with preferred status 1). A 1-signal at the setting input S1 sets the flipflop, i. e. the value of Q1 becomes 1. The value which applies at reset input R is immaterial. A 1-signal at reset input R only brings output Q1 to value 0, if set input S1 is also 0. The set input with this flipflop is therefore dominant.

TP301 • Festo Didactic

B-146 Chapter 12

Function block RS Fig. B12.14: Function block RS, primarily resetting

RS BOOL

S

BOOL

R1

Q1

BOOL

The standard function block RS contains a dominant resetting flipflop (bistable memory with preferred status 0). A 1-signal at the reset input R1 resets the flipflop, i.e. the value of Q1 becomes 0. The value which applies at the setting input S is immaterial. The following example illustrates the use of elementary memory operations. Example

Fig. B12.15: Declaration of variables

If sensor B1 has a 1-Signal, this indicates an error status in the system. A horn H1 is sounded. The horn can only be switched off by actuating push-button S1. It is possible to switch off the horn, even if the B1- signal continues to be applied. VAR B1 AT %IX1 S1 AT %IX2 H1 AT %QX1 RS_H1

: : : :

BOOL; BOOL; BOOL; RS;

(* (* (* (* (*

Sensor detects error status Push button Horn Flip-flop named RS_H1 for Status of horn

*) *) *) *) *)

END_VAR

In the languages FBD and ST, memory operations are realised by invoking a copy of the RS function block. The copy in this example has the name RS_H1. The invocation in FBD is effected by means of graphically linking the current transfer parameters with the inputs of the function block copy. Since the value of the function block copy is relevant, the output of the function block copy is connected correspondingly.

TP301 • Festo Didactic

B-147 Chapter 12

a) LD

Fig. B12.16: Use of function blocks RS

b) FBD B1

S1

H1

RS_H1

S

RS

H1

B1

S

S1

R1

Q1

H1

R

c) IL LD S LD R

d) ST B1 H1 S1 H1

RS_H1 (S := B1, R1 := S1); H1 := RS_H1.Q1;

or CAL LD ST

RS_H1 (S := B1, R1 := S1) RS_H1.Q1 H1

In the textual language ST, the invocation is effected by means of specifying the function block copy. The current parameters are also listed simultaneously. The value of the output of the function block copy RS_H1 can be accessed via the variable RS_H1.Q1; the name of the output variable is therefore defined via the names of the function block copy and the names of the desired output. The languages LD and IL have their own operations for stored setting or resetting of variables, whereby the use of an RS function block can be omitted. It should be noted that the sequence of set and reset commands is crucial for the behaviour of the PLC. The command, which is to be dominant – in this instance the reset command – must only occur after the set command in the program, so that it is the last command to be executed and thereby determines the behaviour – in this case the output.

TP301 • Festo Didactic

B-148 Chapter 12

12.4 Edge evaluation Signals reaching the inputs via sensors are evaluated as 0 or 1 signals by the central control unit of the PLC, whereby the duration of signal statuses 0 and 1 is determined by the sensor. For instance: A 1-signal applies for as long as a push button is pressed, otherwise a 0-signal is received. In many cases, however, it is not the signal itself which matters, but the exact instant, during which the signal changes. This type of signal change is termed Edge. Example

To elucidate this, imagine the switches (push buttons) of a lighting system, where the edge evaluation is mechanically implemented. By actuating the push button, the light comes on (irrespective of how long this push button is pressed). If the push button has been released in the meantime, the light may be switched of by pressing the push button again. In exactly the same way, the moment in which the input signal changes from 0 to 1 must be registered in a PLC, since only ever one single reaction per push button actuation may be triggered (196 irrespective of how long the 1-signal applies. This prevents a process from being put in motion repeatedly by the controller, should a push button be actuated for too long. The edges of the input signal are evaluated for each program. In this context it is referred to as edge detection. Each binary signal has a rising and a falling edge:

Fig. B12.17: Rising and falling edge

rising edge

rising edge

1 0 falling edge

falling edge

Rising edges mark the instants, in which a signal level changes from 0 to 1. Falling edges mark the exact instants, when a signal level changes from 1 to 0.

TP301 • Festo Didactic

B-149 Chapter 12

Whether rising or falling edges are evaluated within a program or function block depends on how the respective sensor is wired (normally closed/normally open contact) and how it is used. A push button (normally open contact), for instance, creates a rising edge the moment it is pressed and a falling edge the moment it is released. EN 61131-3 (IEC 61131-3) provides two standard function blocks for the evaluation of edges Function block R_TRIG, rising edge The standard function block R_TRIG (rising) is used for the detection of rising or positive edges. Its output Q has the value 1 from one execution of the function block to the next, if its input CLK (Clock for pulse) changes from 0 to 1. Fig. B12.18: Function block R_TRIG, rising edge

R_TRIG BOOL

CLK

Q

BOOL

Function block F_TRIG, falling edge A falling or negative switching edge is detected by means of the function block F_TRIG (falling). If a change has taken place at input CLK from 1 to 0, output Q assumes the value 1 for one processing cycle. Fig. B12.19: Function block F_TRIG, falling edge

F_TRIG BOOL

CLK

Q

BOOL

The following example shows the programming of the edge evaluation in the languages FBD, LD, IL and ST, whereby the rising edges are evaluated.

TP301 • Festo Didactic

B-150 Chapter 12

Example

Fig. B12.20: Declaration of variables

Actuation of a push button S1 causes the door of a furnace to be opened. A repeat actuation of push button S1 causes the door to be closed. VAR S1 AT %IX1 : BOOL; 1Y1 AT %QX1 : BOOL;

(* (* (* : RS; (* (* : R_TRIG; (* (* (*

RS_1Y1 R_TRIG_S1

Switch for door Coil for actuation of cylinder for door Flip-flop named RS_1Y1 for status of coil Function block named R_TRIG_S1 for detection of edge at S1 *)

*) *) *) *) *) *) *)

END_VAR

Fig. B12.21: Use of function blocks R_TRIG

a) LD S1

1Y1

1Y1

P

/

S

S1

1Y1

1Y1

P

R

b) FBD

RS_1Y1 R_TRIG_S1

1Y1

&

R_TRIG S1

CLK

RS S

Q

Q1

1Y1

R1 & 1Y1

c) IL CAL LD ANDN S LD AND R

R_TRIG_S1 (CLK := S1) R_TRIG_S1.Q 1Y1 1Y1 R_TRIG_S1.Q 1Y1 1Y1

d) ST R_TRIG_S1 (CLK := S1); RS_1Y1 (S := R_TRIG_S1.Q & NOT 1Y1, R1 := R_TRIG_S1.Q & 1Y1); 1Y1 := RS_1Y1.Q1;

TP301 • Festo Didactic

B-151 Chapter 12

In the languages FBD, IL and ST, edge detection is effected by means of invoking a R_TRIG function block. The name of the function block used in the example is R_TRIG_S1; R_TRIG_S1 represents a copy of the function block type R_TRIG. The LD language has special contacts for the evaluation of edges, whereby the invocation of an R_TRIG function block is omitted.

TP301 • Festo Didactic

B-152 Chapter 12

TP301 • Festo Didactic

B-153 Chapter 13

Timers 13.1 Introduction Many control tasks require the programming of time. For example, cylinder 2A1 is to extend, if cylinder 1A1 is retracted – but only after a delay of 5 seconds. This is known as a switch-on signal delay. Switch-on signal delays during the switching on of power sections is often also required for reasons of safety. The timers of a PLC are realised in the form of software modules and are based on the generation of digital timing. The counted clock pulses are derived from the quartz generator of the microprocessor. The desired time duration is set in the control program. EN 61131-3 (IEC 61131-3) defines three types of timer function blocks:


TP Pulse timing




TON On-delay timing




TOF Off-delay timing

Time duration is specified by means of a defined character format. A time specification is introduced by the characters T# or t#, followed by the time elements, i.e. days, hours, minutes, seconds, milliseconds.

d

Day

h

Hour

m

Minute

s

Second

ms

Millisecond

The following represent examples of permissible time specifications:

T#2h15m T#20s T#10M25S t#3h_40m_20s

Details regarding time specifications may be found in chapter 6.2.

TP301 • Festo Didactic

B-154 Chapter 13

13.2 Pulse timer The function block TP (timer pulse) is a pulse timer, which is started by a shorter or longer 1-signal at input IN. A 1-signal now applies at output Q for the time specified at its input PT (preset time). The output signal Q therefore has a fixed duration, which can be specified by means of a time specification. It cannot be started again while the pulse timer is active. The current time value of the pulse timer is available at output ET (estimated time). Fig. B13.1: Function block TP, pulse timer

Fig. B13.2: Timing diagram of pulse timer TP

TP BOOL

IN

Q

BOOL

TIME

PT

ET

TIME

IN PT

ET 0

Q

The use of a pulse timer is represented with the help of an example. Example

Pressing of the start button S2 is to cause the piston of a cylinder to advance. This mechanism is to be used to clamp workpieces. When the piston has advanced fully, it is to remain in this position for 20 seconds. The cylinder then returns to its initial position

TP301 • Festo Didactic

B-155 Chapter 13

VAR S2 AT %IX1 1B1 AT %IX2 1B2 AT %IX3 1Y1 AT %QX1 SR_1Y1

: : : : :

BOOL; BOOL; BOOL; BOOL; SR;

TP_1Y1

: TP;

(* (* (* (* (* (* (* (*

Start button Cylinder retracted Cylinder advanced Cylinder advance Flipflop named SR_1Y1 for status of 1Y1 TP function block named TP_1Y1

*) *) *) *) *) *) *) *)

Fig. B13.3: Declaration of variables

END_VAR

Fig. B13.4: Use of pulse timer in FBD &

S2

SR_1Y1

1B1

SR S1

TP_1Y1 IN

Q

T#20s

PT

ET

1Y1

R

TP 1B2

Q1

&

1B2

The control task has been programmed in the language FBD as an example. A timer function block may of course be used in any of the other languages. An example using a switch-off delay is given in chapter 13.4 to demonstrate this for the languages FBD, LD, IL and ST. The valve 1Y1 for the actuation of the cylinder is switched via an SR flipflop SR_1Y1. The set condition for SR_1Y1 is met, if the start button for the retracted cylinder is actuated. As soon as the cylinder has advanced, the pulse timer TP_1Y1 with the time of 20 seconds is started by the rising edge of sensor 1B2. Output Q of TP_1Y1 now assumes a 1signal. When the pulse timer has expired – the 20 seconds have passed – 0 applies at output Q of TP_1Y1. The reset condition for SR_1Y1 is fulfilled: the cylinder retracts again. Formulations such as "pulse timer" with the name TP_1Y1" mean: TP_1Y1 is a copy of function block type TP, in this case a copy of the pulse timer.

TP301 • Festo Didactic

B-156 Chapter 13

13.3 Switch-on signal delay The function block TON (timer on-delay) is used to generate switch-on signal delays. After the start via a 1-signal at input IN, output Q does not assume value 1 until the time specified at input PT has expired, and retains this until input signal IN returns to 0. If the duration of the input signal IN is shorter than the specified time PT, the value of the output remains at 0. Fig. B13.5: Function block TON, Switch-on signal delay

Fig. B13.6: Timing diagram of Switch-on signal delay TON

TON BOOL

IN

Q

BOOL

TIME

PT

ET

TIME

IN PT

ET 0

Q

Example

Cylinder 1A1 extends, if start button S1 is actuated. Once this has been extended for 2 seconds, a second cylinder 2A1 moves to its forward end position. Sensors 1B1 and 1B2 indicate the retracted and the forward end positions of cylinder 1A1.

TP301 • Festo Didactic

B-157 Chapter 13

VAR S1 AT %IX1 1B1 AT %IX2 1B2 AT %IX3 1Y1 AT %QX1 2Y1 AT %QX2 RS_1Y1 TON_2Y1

: : : : : : :

BOOL; BOOL; BOOL; BOOL; BOOL; RS; TON;

(* (* (* (* (* (* (* (*

Start button Cylinder 1A1 retracted Cylinder 1A1 advanced Cylinder 1A1 advance Cylinder 2A1 advance Flip-flop named RS_1Y1 for 1Y1 Switch-on signal delay named TON_2Y1 for 2Y1

*) *) *) *) *) *) *) *)

Fig. B13.7: Declaration of variables

END_VAR

Fig. B13.8: Use of Switch-on signal delay in FBD

RS_1Y1 RS &

S1

S

Q1

1Y1

R1

1B1

TON_2Y1 TON 1B2

IN

Q

T#2s

PT

ET

2Y1

Cylinder 1A1 is controlled via valve 1Y1. As soon as cylinder 1A1 has extended and sensor 1B2 has a 1-signal, the switch-on signal delay TON_2Y1 is started. On expiry of 2 seconds, a 1-signal is applied at output Q of TON_2Y1, and cylinder 2A1 extends. Cylinder 2A1 remains extended so long as the 1-signal is applied at input IN of TON_2Y1, i. e. so long as cylinder 1A1 remains extended. As illustrated by this example, not all inputs and outputs of a function block need be connected or supplied. If an input of a function block is not connected – in this case the R1 input of RS_1Y1 – the value of the input from the previous invocation is used. In this case, the initialisation value of the variable R1, which represents a boolean variable, is therefore preallocated with the value 0, i.e. function block RS_1Y1 operates with the value 0 for parameter R1 during its invocation.

TP301 • Festo Didactic

B-158 Chapter 13

13.4 Switch-off signal delay TOF (timer off-delay) is the name of the function block for a Switch-off signal delay. The timer is started via a 1-signal at input IN. At the same time, the output signal Q assumes the value 1. After the input signal IN has reverted to the value 0, the output remains at 1 for the duration PT and does not return to the value 0 until this has expired. Fig. B13.9: Function block TOF, Switchoff signal delay

Fig. B13.10: Timing diagram of Switch-off signal delay TOF

TOF BOOL

IN

Q

BOOL

TIME

PT

ET

TIME

IN PT

ET 0

Q

TP301 • Festo Didactic

B-159 Chapter 13

The following example illustrates the use of a switch-off signal delay in the languages FBD, LD, IL and ST. Following the actuation of a push button, the cylinder of a stamping device is to extend momentarily. When the push button is released, the cylinder is to retract only after a stamping period of 30 seconds. VAR S1 AT %IX1 : BOOL; (* 1Y1 AT %QX1 : BOOL; (* TOF_1Y1 : TOF; (* (* END_VAR

Switch Cylinder advance Switch-off signal delay named TOF_1Y1 for 1Y1

a) LD

*) *) *) *)

TOF_1Y1 TOF IN T#30s

PT

TOF_1Y1 1Y1

Q ET

TOF S1

IN

Q

T#30s

PT

ET

1Y1

c) IL CAL LD ST

TOF_1Y1 (IN := S1, PT := T#30s) TOF_1Y1.Q 1Y1

d) ST TOF_1Y1 (IN :=S1, PT := T#30s); 1Y1 := TOF_1Y1.Q;

In all the languages, a copy of the TOF function block TOF_1Y1 is invoked to realise the switch-off signal delay of the stamping cylinder.

TP301 • Festo Didactic

Fig. B13.11: Declaration of variables

Fig. B13.12: Use of switch-off signal delay in FBD

b) FBD

S1

Example

B-160 Chapter 13

In the LD language, the function block is attached in a current rung via the boolean start input IN and the boolean output Q. If the normally open contact S1 supplies a 1-signal, a 1-signal also applies at output Q of TOF_1Y1. The value Q is copied to the variable 1Y1. As soon as the 1signal of S1 returns to 0, 1 still applies at output Q of TOF_1Y1 for the period of 30 seconds, a 0-signal is supplied subsequently. In the textual languages IL and ST, the switch-off signal delay is invoked by specifying the name TOF_1Y1 of the declared copy and listing the relevant transfer parameters. The status of the switch-off signal delay can be obtained via output Q. In the example given here, the status of the switch-off signal delay TOF_1Y1 is stored in the variable TOF_1Y1.Q.

TP301 • Festo Didactic

B-161 Chapter 14

Counter 14.1 Counter functions Counters are used to detect piece numbers and events. Controllers frequently need to operate with counters in practice. A counter is for instance required, if exactly 10 identical parts are to be conveyed to an conveyor belt via a sorting device. EN 61131-3 (IEC 61131-3) differentiates between three different counter modules:


CTU: Incremental counter




CTD: Decremental counter




CTUD: Incremental/Decremental counter

These standard function modules are used to detect standard, non timecritical counting. With many control tasks it is however necessary to use so-called highspeed counters. "High-speed" in this case generally refers to a counter frequency in excess of 50 Hz, i. e. more than 50 events are counted per second. Tasks of this type cannot be solved with the standard counter function modules of a PLC. The limitations of counter frequency in counter function blocks are due to the output signal delays. Each input signal – i.e. also the counter signal – is delayed by a certain time, before it is released for processing in the PLC. This prevents interference. A further limitation is the cycle time of the PLC. This is why additional counter modules are generally available for PLCs for high-speed counting. High-speed counters are for instance used for the positioning of workpieces. 14.2 Incremental counter The Incremental counter is known as a CTU (count up). The counter is set at the initial value 0 by a signal at reset input R.

TP301 • Festo Didactic

B-162 Chapter 14

Fig. B14.1: Function block CTU, Incremental counter

CTU BOOL

CU

BOOL

R

INT

PV

Q CV

BOOL INT

This current counter status is available at output CV (current value). The value in the counter is then increased by 1 with each positive edge at counter input CU (count up). At the same time the current value is compared in the function block with the preselect value PV. As soon as the current value CV is equal to or greater than the preselect value, the output signal assumes the value 1. Prior to reaching this value, output Q has a 0-signal. The following example demonstrates the use of an incremental counter in the languages FBD, LD, IL and ST. Example

Fig. B14.2: Declaration of variables

Parts are to be ejected from a gravity-feed magazine via a cylinder. IF push button S1 is actuated, the cylinder is to advance, eject a workpiece and then retract again. 15 parts are to be ejected in this way. When 15 parts have been ejected, it should no longer be possible to trigger a cylinder movement via push button S1. First the counter must be reset by actuating push button S2. VAR S1 AT %IX1 S2 AT %IX2 1B1 AT %IX3 1B2 AT %IX4 1Y1 AT %QX1 1Y1_advance AT %MX1 CTU_1Y1_M AT %MX2 RS_1Y1 CTU_1Y1

: BOOL; (* : BOOL; (* (* : BOOL; (* : BOOL; (* : BOOL; (*

Push button for cylinder movement Push button for resetting of counter CTU_1Y1 Cylinder retracted Cylinder advanced Cylinder advance

*) *) *) *) *) *)

: BOOL; (* stores condition: advance cylinder

*)

: BOOL; (* stores the counter status CTU_1Y1 : RS; (* Flip-flop named RS_1Y1 for 1Y1 : CTU; (* Incremental counter named CTU_1Y1 for (* cylinder movements

*) *) *) *)

END_VAR

TP301 • Festo Didactic

B-163 Chapter 14

S1

a) LD

1B1 CTU_1Y1.Q

Fig. B14.3: Use of incremental counter

1Y1 S

/ 1B2

1Y1 R CTU_1Y1

1B2

CTU_1Y1_M

CTU CU

Q

S2 R

15

PV

CV RS_1Y1

S1

b) FBD

&

RS S

1B1 CTU_1Y1.Q

Q1

1Y1

R1

1B2 CTU_1Y1 CTU 1B2

CU

S2

R

15

PV

Q CV

c) IL

LD AND ANDN ST CAL LD ST CAL

d) ST

RS_1Y1 ( S := S1 & 1B1 & NOT CTU_1Y1.Q, R1 := 1B2); 1Y1 := RS_1Y1.Q1; CTU_1Y1 (CU := 1B2, R := S2, PV := 15);

TP301 • Festo Didactic

S1 1B1 CTU_1Y1.Q 1Y1_advance RS_1Y1 (S := 1Y1_advance, R1 := 1B2) RS_1Y1.Q1 1Y1 CTU_1Y1 (CU := 1B2, R := S2, PV := 15)

B-164 Chapter 14

A CTU function block (incremental counter) is used in all languages to realise the counting function; in the actual example the name of the declared copy is CTU_1Y1. The cylinder is actuated via a valve 1Y1. The valve itself is switched via an RS flip-flop named RS_1Y1. The cylinder advances only if push button S1 is actuated and the cylinder (1B1=1) retracted and not with an expired counter (CTU_1Y1.Q = 0). When the cylinder has reached its forward end position (1B2=1), the value of 1Y1 reverts to 0 and the cylinder retracts again. The cylinder strokes are counted via the counter named CTU_1Y1. The counter has a defined status at the beginning of processing, since all variables are preallocated. This means that if the cylinder is in the initial position and none of the push buttons are actuated, whereby a 0-signal is applied at 1B2 and S2 and thus at the CU and R input; the preselect value PV is 15, the current counter value CV is 0. The counter has therefore not yet expired, output Q has the value 0. Actuation of push button S1 causes the cylinder to extend, the rising edge of 1B2 leads to a counting pulse and the current value CV of CTU_1Y1 is increased by 1. When 15 cylinder movements have been executed, the current counter value CV is equal to the preselect value PV; the counter has expired and this is indicated by the value 1 at output Q. The cylinder will not move until the counter resets, i.e. starts anew. This occurs by actuating push-button S2; the 1-signal at the R_input sets the actual counter value CV at 0, following this a 0-signal applies at output Q. Mention is to be made at this point of a particular feature of the IL language. In IL, transfer parameters for a function block must be individual variables only. Expressions are not permissible. This is why the AND operation of variables S1, 1B1 and CTU_1Y1.Q is copied to the boolean variable 1Y1_advance and these are then used as transfer parameters.

TP301 • Festo Didactic

B-165 Chapter 14

14.3 Decremental counter Function block CTD (count down) is the decremental counter of EN 61131-3 (IEC 61131-3) and represents the counterpart of the incremental counter. The decremental counter with preselect value PV is loaded with a 1signal at input LD (load). During normal operation, each positive edge at input CD (count down) reduces the counter reading. The current counter reading is also available at output CV in this instance. Output Q of function block CTD is 0, until the current counter reading CV becomes less than or equal to 0. Fig. B14.4: Function block CTD, Decremental counter

CTD BOOL

CD

BOOL

LD

INT

PV

Q CV

BOOL INT

The use of a decremental counter is also demonstrated by a small example. A cylinder is moved via a valve 1Y1. The position of the cylinder is signalled via the sensors 1B1 (retracted) and 1B2 (extended). The cylinder is to advance, if push button S1 is pressed. When 10 strokes have been executed in this way, lamp H1 is illuminated and the counter has expired. The counter must be re-loaded with the preselect value, before any cylinder movements can be executed further. This is effected by means of actuating push button S2. VAR S1 AT %IX1 S2 AT %IX2 1B1 AT %IX3 1B2 AT %IX4 1Y1 AT %QX1 H1 AT %QX2 RS_1Y1 CTD_1Y1

: BOOL; (* Push button for Cylinder movement : BOOL; (* Push_button for resetting of (* counter CTD_1Y1 : BOOL; (* Cylinder retracted : BOOL; (* Cylinder advanced : BOOL; (* Cylinder advance : BOOL; (* Lamp : RS; (* Flip-flop named RS_1Y1 for 1Y1 : CTD; (* Decremental counter named CTD_1Y1 for (*the cylinder movements

END_VAR

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*) *) *) *) *) *) *) *) *) *)

Example

Fig. B14.5: Declaration of variables

B-166 Chapter 14

Fig. B14.6: Use of decremental counter in FBD language

RS_1Y1 S1

&

RS S

1B1

Q1

1Y1

R1

CTD_1Y1.Q 1B2 CTD_1Y1 CTD 1B2

CD

S2

LD

10

PV

Q

H1

CV

The valve 1Y1 is switched via an RS function block named RS_1Y1. The set condition is met, when the cylinder is retracted, the counter has not yet expired and push button S1 is actuated. When the cylinder has extended completely, 0 applies again at output Q1 of RS_1Y1. The cylinder strokes are detected via a decremental counter named CTD_1Y1. If the cylinder is in the initial position and none of the push buttons are actuated, the following values are applied at the inputs and outputs at the start of the decremental counter processing: the CD and the LD input have a 0-signal, the value 10 applies at input PV; the current counter value CV is 0, condition CV PV, the value of output QD in accordance with the equation CV < 0: Please note that the function of the decremental counter should be used only after the start value has been loaded to the counter via the command LD.

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Fig. B14.7: Function block CTUD, Incremental/ decremental counter

B-168 Chapter 14

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B-169 Chapter 15

Sequence control systems 15.1 What is a sequence control system Sequence control systems are processes in several, clearly separate steps. The progression from one step to the next depends on the step enabling conditions. One important characteristic is that only ever one step may be active or several steps only if these have been explicitly programmed as steps to be simultaneously executed. Compared with a logic control system, it offers the following advantages:


the program is divided into steps and therefore more clearly arranged and easier to maintain and expand




sequence control systems can easily be programmed graphically in a sequential function chart




error detection in a process-related, graphically represented sequence control is more convenient and conclusive than that possible with logic control systems.

Typical examples for sequence controls are machine controls in the sphere of production technology or receptive controllers in process technology. 15.2 Function chart to IEC 60848 The need for configuration is not immediately indicated in the case of small sequence-oriented controllers, but the need for improved functional descriptions increases with the growing complexity of tasks. Ladder diagrams and statement lists are poorly suited for structured description. Function charts (or also flow charts) were introduced as auxiliary means for top-down analysis and for the representation of processes function charts. The elements used for this type of description and their use have been standardised internationally by IEC 60848. Function charts describe in the main two aspects of a controller in accordance with defined rules:


the actions to be executed (commands)




the sequence of execution

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A function chart is therefore divided into two parts (fig. B15.1). The sequence part represents the time-related execution of the process. The sequence part does not describe the actions to be executed individually. These are contained in the action part of the function chart which, for the example in question, consists of blocks on the righthand side of the steps. Fig. B15.1: Function chart for a test process

0

N

Initial position

Part in lifting bracket 1

L

Color and material definition t = 0.5s

Timer expired 2

S

Lifting cylinder raise

Lifting cylinder up 3

L

Defining thickness t = 1s

Timer expired 4

N

Ejecting cylinder advance

Ejecting cylinder advanced 5

N

Ejecting cylinder retract

Ejecting cylinder retracted 6

S

Lifting cylinder lower

Lifting cylinder down

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B-171 Chapter 15

The following provides a brief explanation of the individual elements used to describe a function chart. Steps A function chart is structured by means of steps. These are represented by blocks and identified with the respective step number. The output status of the controller is identified by the initial step. Each step is assigned actions (commands) containing the actual execution parts of the controller. Fig. B15.2: Elements of function chart

Initial step

1 directed connections

Transition Step

2

Action

Transitions A transition is a link from one step to the next. The logic transition condition associated with the transition is represented next to the horizontal line across the transition. If the condition is met, the transition to the next step takes place and this is then processed by the controller.

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Sequence structures Three basic forms of sequence structure may be created by means of combining the step and transition elements:


Linear sequence




Sequence branch (alternative branch)




Sequence splitting (parallel branch)

Steps and enabling conditions must always alternate irrespective of the form of the sequence structure. Sequence structures are processed from top to bottom. In a linear sequence, only one transition follows a step and one step each transition. Fig. B15.1 illustrates an example of a linear sequence. Fig. B15.3 Alternative branch

2 d

g 5

3 e

h

4 f 6 i

In the alternative branch shown in fig. B15.3, two or several transitions follow a step. The partial sequence, whose transition condition has been met first, is activated and processed. Since precisely one partial sequence may be selected with the alternative branch, the transition conditions – d and g in fig. B15.3 – must be mutually exclusive.

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B-173 Chapter 15

Fig. B15.4: Parallel branch

2 d 5

3 e

f 6

4 g 7

In the case of a parallel branch, of the transition condition is met, this leads to the simultaneous activation of several partial sequences. The partial sequences are evolved simultaneously, but completely independently of one another. The convergence of partial sequences is synchronised. Only when all parallel partial sequences have been evolved, may a transition to the next step underneath the double line – in this example to step 7 – take place. Action Each step contains actions, the actual execution parts of the controller. The action itself (fig. B15.5) is divided into three fields, whereby field a and c should only be represented if necessary.

a

b

c

a: Characterisation of actions to be executed b: Description of action c: Reference to all feedbacks associated with command

Table B15.1 contains the symbols defined in IEC 60848 used to describe the order of execution of the actions.

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Fig. B15.5: Action

B-174 Chapter 15

Table B15.1: Mode of actions

S

stored

N

non-stored

D

delayed

F

enabling

L

limited

P

pulse

C

conditional

If an action needs to be described in more detail, a combination of letter symbols should be selected in the order of this execution. Example

DCSF conditionally stored action after delay, subject to an additional enabling condition after storage. Step refinement As shown in fig. B15.6, each step may itself contain sequence structures. This facility is supported by the step-by-step refinement of a solution in the sense of a top-down design.

Fig. B15.6: Step refinement

2.1

N

Filling magazine

Part in magazine and gripper in position 1 and ejecting cylinder retracted

Start 2

2.2

S

Ejecting cylinder out

Part ejected Release part

2.3

S S

Grip part Gripper to position 2

Part accepted and part release requested

2.4

S S

Part released Ejecting cylinder in

Part released 2.5

S

Gripper in position 1

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B-175 Chapter 16

Commissioning and operational safety of a PLC 16.1 Commissioning PLC programs are never final in that it is always possible to make corrections and subsequent adaptations to new system requirements. Even during commissioning, program changes are often necessary. The commissioning of a system can be divided into basically four steps:


Checking the hardware




Transferring and testing the software




Optimisation of software




Commissioning of the system

Checking the hardware Each sensor is connected to a specific input and each actuator to an output; addresses must not be mixed up. This is why the first step in checking the hardware always comes after the allocation list. Are all the sensors and actuators allocated to the right input and output addresses? Is the function – for 0- and 1-signal – identified uniquely, The allocation list must be correct and completed in full as this forms part of the documentation prior to the commissioning of a program. During checking, the outputs are set by way of a test. The actuators must then meet the functions specified. Transferring and testing of software Even prior to commissioning, all available off-line testing facilities of the program system should be used intensively. One such convenient test function is for instance, the simulation of the program.

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Following this, the program is transferred to the central control unit of the PLC. A small number of PLCs now offer a facility for simulation: The entire program is executed without the inputs and outputs being connected. Similarly, only the connection of the outputs may be omitted. Processing of the PLC outputs thus only takes place in the image table, in that the image table is not switched through to the physically available outputs. This therefore eliminates the risk of damaging machines or system parts, which is of particular importance in the case of dangerous or critical processes. After this, the individual program parts and system functions are tested: Manual operation, setting, individual monitoring programs etc., and finally the interaction of the program parts with the help of the overall program. The system is therefore commissioned step-by-step. Important aspects of commissioning and error detection are test functions of the programming system such as single-step mode or the setting of stop points. Single-step mode in particular is of importance, whereby the program in the PLC memory is executed line-by-line or step-by-step. In this way, any errors which may occur in the program can be immediately localised. Optimisation of software Larger programs can almost always be improved after the first test run. It is important that any corrections or modifications are made not just in the PLC program, but are also taken into account in the documentation. Apart from the documentation, the status of the software has to be saved. Commissioning of the system This already occurs in part during the testing and optimisation phase. Once the final status of PLC program and the documentation is established, all the controller functions (in accordance with the task) need to be executed step for step again. The system is then ready to be accepted by the customer or the relevant department.

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B-177 Chapter 16

16.2 Operational safety of a PLC PLC voltage supply Differentiation must be made between Control voltage (signals between controlled machine and the PLC) and the Logic voltage (for the internal voltage supply of the central control unit). The level of the operating voltage of a PLC is specified in EN 61131-2 (IEC 61131-2). It is between 24VDC and 48VDC or 48VAC and 230VAC respectively. 120VAC may also apply for the American market. Control voltage The control voltage supplies the sensors and actuators. The user needs to connect a power supply unit to the controller for this. In Germany, the control voltage of a PLC is generally 24VDC or 230VAC. (In the main, DC voltage is used.) In other countries, different voltages are also used, e.g. 48VDC voltage or 120VAC voltage. How exactly the power supply unit is connected to the controller varies according to the PLC used. The control voltage permits a certain amount of variation. The PLC modules are in the main protected against excessive voltages, depending on the module through which the central control unit is realised. Logic voltage In addition, a PLC requires a voltage supply for the internal logic: The logic voltage, which forms the signals in the central control unit. This is why the logic voltage must meet very high requirements, i.e., it needs to be stabilised. Either 5 V (TTL level) is used for this or approx. 10 V (CMOS level), depending on the module through which the central control unit is realised. There are three possibilities of Voltage supply: 1. Control voltage and logic voltage are generated completely separately from the mains voltage. 2. Two power supply units combined in one housing for the generation of the two voltages. 3. The logic voltage is generated from the control voltage (not the mains voltage).

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B-178 Chapter 16

Interference suppression All PLCs are extremely sensitive to voltage supply interference. Differentiation should be made between to different versions:


Interferences reaching the logic voltage from the voltage supply via the power supply unit;




Interferences, affecting the lines to and from the sensors and actuators.

1. Interferences in the logic voltage A main interference suppression filter and a capacitor protect against interferences of this type. The mains interference suppression filter protects against overvoltages and interference signals from voltage supply. A capacitor stores electrical energy, whereby the controller voltage supply is protected even in the event of brief voltage failures. If this type of voltage suppression has not been provided by the PLC manufacturer, a mains interference suppression filter and capacitor is to be fitted subsequently by the user. 2. Line interferences to and from sensors and actuators Interference pulses on electrical lines may cause a 1- or 0- signal to occur at PLC inputs, which has not be supplied by a sensor. The signal may be created as a result of effects from other cables. This kind of interference is dangerous: As a rule, input modules of a PLC are therefore protected by means of a series connected optocoupler and signal delay. The optocoupler protects against overvoltages of up to approx. 5000 V. The signal delay prevents spurious signals, since these are generally very brief. The length of time varies between 1 and 20 ms depending on the PLC. “High speed” input modules (without signal delay) must be shielded, for example, by screened cables.

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B-179 Chapter 16

Output modules also contain an optocoupler for protection against overvoltages. Moreover, the outputs are short-circuit protected, though normally not against sustained short-circuit. Mutual induction voltage When inductive actuators (e.g. safety coils, solenoid coils) are switched, it creates a mutual induction voltage at the coil. This mutual induction voltage must be eliminated to protect the output module. A suppressor diode is used for this. The output modules of a number of PLCs are already equipped with suppressor diodes of this type. The residual voltage in this case, however, remains an interference factor on the interconnecting cables. This is why the protective measures should be taken direct at the point of origin, i.e. on the coil: by means of a suppressor diode (for DC voltage only) or a varistor (voltage-dependent resistor). Two reverse polarity and Zener diodes switched parallel to the coil may also be used. With a voltage in excess of 150 V, however, several breakdown diodes must be switched in series. EMERGENCY-STOP If the EMERGENCY-STOP device is actuated, it is essential for a condition to be achieved, which is harmless both for people and system. Final control elements and drives, which may produce dangerous situations, must be switched off immediately (e.g. spindle drives). Conversely, final control elements and drives, which may prove dangerous to people or the system when switched off, must continue operating even in an emergency (e.g. clamping devices). The facility to operate an EMERGENCY-STOP must be available at any given time in a system. This is why a standard electronic controller may not assume the EMERGENCY- STOP function. The EMERGENCY-STOP circuit must be established independently of the PLC by means of contactor technology. EN 418 and EN 954 also specify this, since it would be impossible to switch an EMERGENCY-STOP with a malfunctioning controller. Once the EMERGENCY-STOP device has been unlatched, it should be no longer possible for machinery to operate automatically.

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B-180 Chapter 16

Example

An output transistor of the PLC is burnt out. A voltage of 24 V applies permanently at the output (corresponding to a 1-signal). The solenoid value is actuated; the cylinder extends, even though the system has not been enabled. If the EMERGENCY-STOP command were to be executed by the PLC program, it would remain inactive, since the error only occurs "after" the program. The EMERGENCY-STOP command must therefore take effect downstream of the PLC as far as the cylinder. One method is to connect the EMERGENCY-STOP function to the voltage supply of the output modules. The connection must be fail-safe. In the event of an EMERGENCY-STOP, all outputs assume the 0-signal. It is of no importance whether a particular output has been set or reset by the PLC. If this method is employed, the connected actuators must move into a non hazardous position in the event of a 0-signal! The following actuators should be used if possible: Hydraulic/pneumatic valves: 5/4- or 5/3-way valves with mid-position normally closed are used (poss. with additional clamping cylinder). These valves clamp the cylinder between fluid or air cushions. Correct connection: Short paths from cylinder to valve, restrictors in exhaust air of the valve. Electric motors: Brake motors are used. In the event of voltage failure, the brake comes into full effect as a result of spring force. The EMERGENCY-STOP circuit in the Hardware carries out the actual safety function. In addition, the EMERGENCY-STOP command must also be stored in the PLC program. Whatever has been effected beyond the PLC by way of hardware must be retraced in each program: in this case, switching off the outputs. This is defined in a parallel program. Once the EMERGENCY-STOP has been reset, the system should not be able to start again on its own. A separate push button/switch is to be actuated to start the system.

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B-181 Chapter 16

The re-starting of the system may be controlled by means of the PLC program. There are two methods for starting:


Continuing from the same point;




Returning to the initial position and re-starting the machine.

In the second instance, it is necessary to switch over to manual or setting mode. If additional safety measures are required for EMERGENCY-STOP, user’s own relays or pneumatic controllers must be used. A special safety-oriented PLC may also be used, which operates by means of two separate central control units with two series connected output stages each. Fail-safe connection The majority of machines are switched on by means of one push button and switched off via another push button. The OFF push button additionally assumes a safety function: The working process may be interrupted at any time and the machine stopped. With EMERGENCYSTOP, however, the entire system is switched off. In contrast with EMERGENCY-STOP, the OFF function is controlled via the PLC. It should be noted, however, that the OFF function must be maintained even if the wiring of the OFF button is defective. The connection must be fail-safe; i.e. the OFF button is to be connected and programmed in the form of a normally closed contact. (The allocation list provides information regarding the significance of the 1- and 0-signal!) A signal generator monitors the oil temperature of a gear unit. For safety reasons, the connection of the signal generator is to be fail-safe: The 1signal identifies the correct temperature, the 0-signal the incorrect temperature. If the connection is defective, the signal generator also assumes a 0-signal (even if in this case the cause is not the incorrect temperature). This eliminates the situation, where a critical condition in the system is no longer signalled by the signal generator because of defective wiring.

TP301 • Festo Didactic

Example

B-182 Chapter 16

TP301 • Festo Didactic

B-183 Chapter 17

Communication 17.1 The need for communication By communication, we understand the transfer of information, i.e. data between the programmable logic controller and other data processing devices, whereby these devices are used as an auxiliary means for specific control tasks, e.g. input of data takes place via a computer, output of data via a printer controlling still remains the task of the PLC. Automation increases the need for communication. Data needs to be continually passed on from production to other operational areas. This provides an overview of the production status and the individual tasks (production data acquisition). Automated systems nowadays are equipped with complex error and fault detection systems. Fault indications and warnings must be generated, centralised and communicated automatically to the operator. To this end, a printer – for logging – or an electronic display is connected to the controller. In some cases, data is to be transferred to the PLC by a computer in an active process, or several control devices are combined into one system network. 17.2 Data transmission How can the PLC communicate with other data processing devices? The individual bits, which are combined into one data word, must be transmitted from one piece of data terminal equipment to another. Basic differentiation is made here between two methods: parallel or serial data transmission. Parallel data transmission means that a separate line must be available for each individual binary signal. When signal generators for example, are connected to a programmable logic controller, a separate wire is installed for each push button, limit switch, limiting value encoder and sensor to a terminal strip and from there to the input of the PLC. All information (“push button actuated”, “cylinder advanced”) can in this way be transmitted simultaneously (parallel) to the PLC. Since in the case of parallel transmission of input and output signals, a line is required for each signal generator, literally miles of cable bundles are installed overall for correspondingly complex machines. For the parallel transmission of a data word, sufficient lines must therefore be available to transmit all bits of this data word simultaneously.

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B-184 Chapter 17

With serial data transmission only one binary signal is transmitted at a time. Again, using the example of the PLC: If several modules of a PLC are interconnected, it is not necessary for a individual line to be installed for each input or output, instead the information regarding inputs or outputs is transmitted consecutively (serial). Accordingly only one data line is therefore required for the serial transmission of data words, irrespective of the number of bits, to transmit the binary signals consecutively. In order to now be able to represent the various signals in the form of a related data word, it is necessary to agree the transmission speed, word length and specific start and end characters. 17.3 Interfaces Different coding procedures, transmission and operating methods as well as different methods of data protection make it essential to define electrical, functional and mechanical characteristics of interfaces in standards. Voltage interfaces Table B171: Interfaces

Current interface

Designation

V.24

Centronics

20 mA

Transmission mode

serial asynchronus

parallel

serial asynchronus

Mode of operation

full duplex

simplex

full duplex

Standard

V.24 RS-232-C

Centronics TTL

TTY

Transmission distance, transmission speed

up to 30 m 20 000 Bit/s

up to 2 m 106 Bit/s

up to 1000 m 20 000 Bit/s

Logic level Data line

15 V ≥ ’0’ ≥ 3 V -3 V ≥ ’1’ ≥ -15 V

’1’ ≥ 2,4 V ’0’ ≥ -0,8 V

’1’ = current off ’0’ = current on

A parallel interface is also known as a Centronics interface. 8 data lines are available for data transmission, i.e. 8 bits may be transmitted simultaneously. The Centronics interface is very frequently used – over small distances – for the actuation of printers.

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B-185 Chapter 17

The most frequently used interface for serial data transmission is the V.24 interface. The Centronics and V.24 interface are both voltage interfaces. Bits are represented for ’0’ or ’1’ via a specified voltage level. In order to create this signal level, a joint ground line must be incorporated for the V.24 interface. In the case of a Centronics interface, each data line has its own ground line. In the case of both interfaces, additional lines have been defined for data flow control apart from the data and ground lines. Considerably more simple than via a V.24 interface is a connection configured via a serial 20 mA interface. All this current-loop interface needs is a transmitter and receiver loop for the transmission of data. A constant current of 20 mA signals the ’0’level (logic0), "current off" signals the ’1’level (logic1) on the data line. This interface is widely used in control technology due to its interference immunity. 17.4 Communication in the field area A multitude of information has to be transported within automated systems and machines. Simple binary sensor signals, analogue signals of measuring sensors or proportional valves, and also recorded data and parameters for the control of processes need to be exchanged reliably between the control technology components of an automated system. The data exchange for this must take place within specified reaction times, since system parts could otherwise continue to operate uncontrolled. A fieldbus is a serial, digital transmission system for these signals and data. All stations on a fieldbus must be in a position to receive the communication from other bus stations and to exchange data in accordance with the agreed protocol. A bus station, taking the initiative for the data exchange is known as a master. Bus stations receiving or supplying data purely on the instruction of the master are termed slaves. Two-wire cables consisting of either twisted pairs or coaxial cables are used for the transmission of data in bus systems. The extent of wiring for bus coupled systems is therefore minimal.

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B-186 Chapter 17

A multitude of different bus systems is available in the market place, which can basically be divided into 2 groups: closed and open bus systems. By closed systems we understand systems, which are


vendor-specific,




do not have any transmission protocol disclosure and




are not compatible. Furthermore, they do not permit interfacing with devices of other manufacturers and adaptation associated with high expenditure.

Closed systems, for instance, are SINEC L1 by Siemens, SUCOnet K by Moeller, Data Highway by Allen Bradley, Festo Fieldbus. Open systems, in contrast have


standardised interfaces and protocols,




declared protocols and




a multitude of devices by different manufacturers may be connected to the bus.

Open systems, for instance, are Profibus, Interbus-S, CAN, SINEC L2, ASI. The advantages of networking with open bus systems are as follows:


Decentralisation of control function




Coordination of processes in separate areas




Realisation of control and production data flow parallel to material flow




Simplification of the installation and reduction of wiring costs (twowire bus)




Simplification of the commissioning of a system (greater clarity, pretested subsystems)




Reduction in service costs (central system diagnostics)




Use of equipment by different manufacturers in the same network




Process data transmission right up to planning level

TP301 • Festo Didactic

B-187 Appendix A

Appendix

Bibliography of illustrations

Fig. B1.2:

Example of a PLC: Festo IPC PS1 Professional Beck IPC GmbH Grabenheimer Str. 36.38, 35578 Wetzlar

Fig. B1.4:

Compact PLC (Mitsubishi FX0, Festo IPC FEC Standard), modulare SPS (Siemens S7-300) Mitsubishi Electric Europe GmbH Gothaer Straße 8, 40880 Rathingen Beck IPC GmbH Grabenheimer Str. 36.38, 35578 Wetzlar Siemens AG AUT 111, Postfach 4848, 90475 Nürnberg

TP301 • Festo Didactic

B-188 Appendix A

TP301 • Festo Didactic

B-189 Appendix B

Bibliography of literature

Berger, Hans

Automating with SIMATIC S7-300/400 Wiley-VCH Verlag, 2000

Berger, Hans

Automating with STEP 7 in STL and SCL Wiley-VCH Verlag, 2000

Berger, Hans

Automating with STEP 7 in LAD and FDB Wiley-VCH Verlag, 2000

John, Karl-Heinz Tiegelkamp, Michael

IEC 1131-3: Programming Industrial Automation Systems. Concepts and Programming Languages, Requirements for Programming Languages, Requirements for Programming Systems, Decicion-Making Tools Springer-Verlag, Berlin Heidelberg, 2001

Lewis, Robert

Programming Industrial Control systems using IEC 1131-3 Institution of Electrical Engineers, 1998

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B-190 Appendix B

TP301 • Festo Didactic

B-191 Appendix C

Guidelines and standards

EN 50170

General purpose field communication system

EN 60204 (IEC 60204)

Safety of machinery - Electrical equipment of machines; Part 1: General requirements

EN 60617 (IEC 60617)

Graphical symbols for diagrams; Part 12: Binary logic elements

EN 61131 (IEC 61131)

Programmable controllers; Part 1: general information Part 2: equipment requirements and tests Part 3: programming languages

IEC 60848

Preparation of function charts for control systems

ISO 1219

Fluid power systems and components; graphic symbols and circuit diagrams; Part 1: graphic symbols Part 2: circuit diagrams

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B-192 Appendix C

TP301 • Festo Didactic

B-193 Appendix D

Glossary Access right EN 61131-3 (IEC 61131-3) specifies for all variables, from which point and to which extent read or write or changing variable access may occur. Action Elements of a step. The action contains the execution parts of a program in the sequential function chart. Action block Collection of action qualifier, action name, feedback variables and action content. Action qualifier Each action has a qualifier, which describes its behaviour in greater detail. Address bus Cable bundle of a system, which exclusively transmits address information, e. g. addresses of PLC input or outputs, flags or peripheral devices. Allocation list, declaration list The allocation list designates and explains the equipment connected to a PLC (push buttons, valves, contactors etc.). In the EN 61131-3 (IEC 61131-3), these lists are replaced by the structured declaration of variables. This does not mean that a program system would not be able to create lists of this type for the documentation of a control program. Alternative branch Branch in the execution of a program in sequential function chart into one of several possible paths. The path taken depends on the transition conditions of the program runtime. Bit Derived from ’binary digit’: bivalent (binary) character, smallest unit of information.

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B-194 Appendix D

Bus system Bus system for the transmission of data between for example, individual PLC modules. Byte A unit of data consisting of 8 bits. Command, instruction Instruction to the MPU of a PLC for the execution of a precisely defined operation. The command consists of an operation part, which contains the information of what is to be done, and the operand or address part. This defines, where something is to be done and where information is to be stored or read. Control bus Bus circuit of a bus system, which exclusively transmits control instructions. Control program The program of a PLC representing the total of all instructions for signal processing, as a result of which the equipment to be controlled is influenced. It consists of a sequence of instructions. Cycle In a cycle, the status of the inputs is read to the image table at the beginning, the program completely processed once, followed by the mage table written to the outputs. Cycle time The cycle time of the PLC is the time required by the central control unit to process a program once from beginning to end. This also takes into account the time required to read and for output of the image table. The time specification is generally in ms/K code. Data bus Bus of a PLC, with which information (signals, data) are transmitted. Data type The variables of EN 61131-3 (IEC 61131-3) may be of different types. Corresponding declarations permit the definition of data types.

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B-195 Appendix D

Declaration of data To be able to access data with symbolic names, these must first be linked in declarations with data types. EEPROM Further development of EPROMs. An electrical impulse is used to erase the contents of a memory, instead of UV light. Subsequently, new information may be written to the memory. EPROM Read-only memory, whose content is erased with UV light, and which can then be written to with new information. Feedback variable The feedback variable is programmed by the user and indicates the end or also an error condition of the action. The feedback variable is frequently used in the subsequent transition condition. Functions A function is a program part, which supplies a unique value for the transmitted input variables. Information cannot be stored intermediately in a function. Function blocks Function blocks are program parts which can be used repeatedly to, process the input data into output data. They can also be used to store intermediate results and have one name for each application. Function block diagram (FBD) An EN 61131-3 (IEC 61131-3) programming language, which graphically interconnects the functions and function blocks in order to solve a control task. Image table PLC programs as a rule do not operate directly on the inputs and outputs of the PLC, but on an image (copy of the signal values in the internal memory). Inputs are read at the beginning of a cycle and at the end of the cycle the image of the outputs is physically output to the outputs.

TP301 • Festo Didactic

B-196 Appendix D

Initial step, Initialisation step A special step in a sequential function chart program, which is first executed at the start of the program. Each sequence structure (network) has exactly one initial step. Initial value, Start value A variable may be assigned an initial value during the program generation, which the variable is to assume with the initial start of the program. Instruction list (IL) Machine-related programming languages to EN 61131-3 (IEC 61131-3), made up of instructions. Instructions consist mainly of an operator and an operand. Key word Certain words in the standard are reserved for specific tasks and therefore cannot be used freely. Ladder diagram (LD) A graphic programming language standardised in EN 61131-3 (IEC 61131-3), derived from the representation of a circuit diagram. Logic control system Control system, in which output signals are assigned to the input signals at any time. This assignment is effected predominantly by means of logic operations. A logic program is fully implemented in one cycle. MPU Abbreviation for Main Processing Unit (formerly CPU) Parallel (simultaneous) branch Parallel branches are those types of branches in a sequential function chart, which contain a parallel program continuation onto several paths simultaneously. When reaching the end, their completion is synchronised. Parallel data transmission The transmission of each individual byte is bit parallel , i. e. an individual, separate data line is available for each bit and the 8 bits are transmitted simultaneously.

TP301 • Festo Didactic

B-197 Appendix D

Parallel interface Interface for bit parallel data transmission. Advantage: High transmission rates are achieved as a result of the simultaneous transmission of 8 data bits. Disadvantage: limited line length. Programming languages EN 61131-3 (IEC 61131-3) permits the use of different programming languages, partly even in mixed form, for the programming of a control program. Function block diagram, the languages ladder diagram, instruction list and structured text are standardised programming languages of this type. Program organisation units EN 61131-3 (IEC 61131-3) differentiates between different parts of a control program and requires structuring into organisation units. These are programs, function blocks and functions. A higher-level organisation unit is the configuration. Read-only memory Read-only-memory (ROM): Memory, whose content may only be read. Sequence control system A control system, which is subdivided into consecutively processed steps. Only after a step has been executed, is the next one executed. The step enabling from one step to the next depends on certain conditions, the step enabling conditions. Sequential function chart A graphic programming language of EN 61131-3 (IEC 61131-3). Its aim is to structure the tasks of the control program into steps and transitions. Serial data transmission Form of data transmission, whereby data is transmitted consecutively bit by bit. At least one data line is required for this. Serial interface Interface for serial data transmission. Advantage: A serial interface permits data to be transmitted and received simultaneously. Disadvantage: Low transmission rates.

TP301 • Festo Didactic

B-198 Appendix D

Step Steps are the execution parts and at the same time organisation resources of the sequential function chart. Structured text (ST) A textual high-level language of EN 61131-3 (IEC 61131-3) for the programming of control programs. Transition The element of a sequential function chart, which initiates the evaluation of a boolean variable or a boolean expression. The result is used to organise the program flow..

TP301 • Festo Didactic

B-199 Appendix E

Index A

Accumulator

B-35

Action blocks

B-124

Actions

B-124

Address line

B-34

Addressing Symbolic

B-61

Alternative branch Application program memory EEPROM EPROM RAM Assignments B

B-117 B-39 B-40 B-40 B-39 B-102

Basic functions, logic AND-function further logic operations NOT-function OR-function

B-19 B-20 B-23 B-19 B-22

Basic logic functions

B-19

BCD-Code

B-13

Boolean algebra rules

B-19 B-19

Boolean equation

B-19

Branch alternative parallel

B-117 B-119

Bus systems

B-185

TP301 • Festo Didactic

B-200 Appendix E

C

D

CASE statement

B-105

Central control unit

B-43

Command

B-35

Commissioning

B-175

Communication in the field area

B-183 B-185

Connection fail-safe

B-180

Control line

B-34

Control unit

B-36

Control voltage

B-177

Counters decremental incremental incremental/decremental

B-165 B-161 B-167

Counter functions

B-161

Data line

B-184

Data transmission parallel seriel

B-183 B-183 B-184

Data types

B-60, B-64

Decremental counter

B-165

Delay switch-off signal switch-on signal

B-158 B-156

Diagnostic tool

B-45

Documentation

B-50

TP301 • Festo Didactic

B-201 Appendix E

E

Edge

B-148

Edge evaluation

B-148

EEPROM EMERGENGY-STOP EPROM EXIT statement Expression F

Fail-safe connection FBD

B-179 B-40 B-109 B-99 B-180 see function block diagram

Fieldbus

B-185

Firmware

B-33

FOR loop

B-106

Function block diagram Elements of Function block F_TRIG R_TRIG RS SR Function blocks standard user-defined Function chart Functions, logic simplification H

B-40

Hardware

TP301 • Festo Didactic

B-85 B-85 B-149 B-149 B-146 B-145 B-77 B-80 B-82 B-169 B-28 B-33

B-202 Appendix E

I

IF statement IL

B-104 see Instruction list

Incremental counter

B-161

Incremental/decremental counter

B-167

Initialisation

B-69

Input module

B-41

Input signal delay

B-42

Instruction list

B-95

Instruction register

B-35

Instructions

B-95

Interfaces

B-184

Interference suppression

B-178

Iteration statements FOR loop REPEAT loop WHILE loop

B-106 B-106 B-107 B-108

K

Karnaugh-Veitch diagram

B-30

L

Label

B-95

Ladder diagram Elements of

B-89 B-89

LD

M

see Ladder diagram

Logic control systems

B-147

Logic voltage

B-177

Microcomputer Multitasking Mutual induction voltage

B-34 B-4 B-179

TP301 • Festo Didactic

B-203 Appendix E

N

O

Network

B-86

von-Neumann principle

B-36

Number systems binary decimal hexadecimal

B-11 B-11 B-13

Operand

B-99

Operational safety of a PLC

P

B-175

Operator

B-99

Optocoupler

B-41

Output module permissible power

B-43 B-44

Overlead protection

B-44

Parallel Branch

B-119

Parallel data transmission

B-183

Personal computer Phase model PLC Areas of application of Basic design

B-45 see PLC software generation B-2 B-5

PLC program cycle time cyclical processing

B-38 B-38

PLC software generation

B-47

Power amplification

B-44

Precedence Priority

B-100 B-99, B-118

Process image

B-38

Program counter

B-36

Program organisation units

B-70

Programming device

B-45

Programming languages

B-54

Programs

B-83

TP301 • Festo Didactic

B-204 Appendix E

Pulse timer Timing diagram R

RAM

B-39

Real numbers

B-14

Relays

B-44

REPEAT loop

S

B-154 B-154

B-107

Representation of data numerical data strings time data

B-61 B-62 B-63 B-62

Resources of a PLC Designation

B-60

Rung

B-93

Screening of input signals

B-42

Selection statements CASE IF

B-103 B-105 B-104

Sequence control systems

B-169

Sequential function chart

B-111

Serial data transmission

B-184

SFC

see Sequential function chart

Short-circuit protection

B-44

Signals binary digital generation of binary and digital voltage range

B-15 B-16 B-15 B-15

Signal detection

B-41

Software

B-33

ST Standard form conjunctive disjunctive

see Structured text B-27 B-27

TP301 • Festo Didactic

B-205 Appendix E

Standards IEC 61131

B-8

Statement list

B-49

Steps

T

Structured text

B-99

Structuring resources at configuration level at program level

B-50 B-52 B-51

Switch-off signal delay timing diagram

B-158 B-158

Switch-on signal delay timing diagram

B-156 B-156

Switching functions simplification of

B-25

Symbolic addressing

B-61

Timer Transition Transition condition

V

B-153 B-115, B-120 B-121

Truth table

B-19, B-26

Value table

see truth table

Variable declaration

B-65

Variables directly addressed

B-60 B-60

Voltage adjustment

W

B-113, B-116

B-41, B-43

Voltage supply

B-177

WHILE loop

B-108

TP301 • Festo Didactic

B-206 Appendix E

TP301 • Festo Didactic